diff --git a/lerobot/common/datasets/aloha.py b/lerobot/common/datasets/aloha.py
index 639acf1f..1fe27e95 100644
--- a/lerobot/common/datasets/aloha.py
+++ b/lerobot/common/datasets/aloha.py
@@ -105,7 +105,7 @@ class AlohaDataset(torch.utils.data.Dataset):
 
     @property
     def num_samples(self) -> int:
-        return len(self.data_dict["index"])
+        return len(self.data_dict["index"]) if "index" in self.data_dict else 0
 
     @property
     def num_episodes(self) -> int:
diff --git a/lerobot/common/datasets/pusht.py b/lerobot/common/datasets/pusht.py
index b468637e..068b154e 100644
--- a/lerobot/common/datasets/pusht.py
+++ b/lerobot/common/datasets/pusht.py
@@ -119,7 +119,7 @@ class PushtDataset(torch.utils.data.Dataset):
 
     @property
     def num_samples(self) -> int:
-        return len(self.data_dict["index"])
+        return len(self.data_dict["index"]) if "index" in self.data_dict else 0
 
     @property
     def num_episodes(self) -> int:
diff --git a/lerobot/common/datasets/xarm.py b/lerobot/common/datasets/xarm.py
index 733267ab..0dfcc5c9 100644
--- a/lerobot/common/datasets/xarm.py
+++ b/lerobot/common/datasets/xarm.py
@@ -60,7 +60,7 @@ class XarmDataset(torch.utils.data.Dataset):
 
     @property
     def num_samples(self) -> int:
-        return len(self.data_dict["index"])
+        return len(self.data_dict["index"]) if "index" in self.data_dict else 0
 
     @property
     def num_episodes(self) -> int:
@@ -126,7 +126,8 @@ class XarmDataset(torch.utils.data.Dataset):
             image = torch.tensor(dataset_dict["observations"]["rgb"][idx0:idx1])
             state = torch.tensor(dataset_dict["observations"]["state"][idx0:idx1])
             action = torch.tensor(dataset_dict["actions"][idx0:idx1])
-            # TODO(rcadene): concat the last "next_observations" to "observations"
+            # TODO(rcadene): we have a missing last frame which is the observation when the env is done
+            # it is critical to have this frame for tdmpc to predict a "done observation/state"
             # next_image = torch.tensor(dataset_dict["next_observations"]["rgb"][idx0:idx1])
             # next_state = torch.tensor(dataset_dict["next_observations"]["state"][idx0:idx1])
             next_reward = torch.tensor(dataset_dict["rewards"][idx0:idx1])
diff --git a/lerobot/common/policies/factory.py b/lerobot/common/policies/factory.py
index 8636aa6e..98880f4a 100644
--- a/lerobot/common/policies/factory.py
+++ b/lerobot/common/policies/factory.py
@@ -35,9 +35,9 @@ def make_policy(cfg):
     if cfg.policy.pretrained_model_path:
         # TODO(rcadene): hack for old pretrained models from fowm
         if cfg.policy.name == "tdmpc" and "fowm" in cfg.policy.pretrained_model_path:
-            if "offline" in cfg.pretrained_model_path:
+            if "offline" in cfg.policy.pretrained_model_path:
                 policy.step[0] = 25000
-            elif "final" in cfg.pretrained_model_path:
+            elif "final" in cfg.policy.pretrained_model_path:
                 policy.step[0] = 100000
             else:
                 raise NotImplementedError()
diff --git a/lerobot/common/policies/tdmpc/policy.py b/lerobot/common/policies/tdmpc/policy.py
index 942ee9b1..14728576 100644
--- a/lerobot/common/policies/tdmpc/policy.py
+++ b/lerobot/common/policies/tdmpc/policy.py
@@ -333,94 +333,6 @@ class TDMPCPolicy(nn.Module):
         """Main update function. Corresponds to one iteration of the model learning."""
         start_time = time.time()
 
-        # num_slices = self.cfg.batch_size
-        # batch_size = self.cfg.horizon * num_slices
-
-        # if demo_buffer is None:
-        #     demo_batch_size = 0
-        # else:
-        #     # Update oversampling ratio
-        #     demo_pc_batch = h.linear_schedule(self.cfg.demo_schedule, step)
-        #     demo_num_slices = int(demo_pc_batch * self.batch_size)
-        #     demo_batch_size = self.cfg.horizon * demo_num_slices
-        #     batch_size -= demo_batch_size
-        #     num_slices -= demo_num_slices
-        #     replay_buffer._sampler.num_slices = num_slices
-        #     demo_buffer._sampler.num_slices = demo_num_slices
-
-        #     assert demo_batch_size % self.cfg.horizon == 0
-        #     assert demo_batch_size % demo_num_slices == 0
-
-        # assert batch_size % self.cfg.horizon == 0
-        # assert batch_size % num_slices == 0
-
-        # # Sample from interaction dataset
-
-        # def process_batch(batch, horizon, num_slices):
-        #     # trajectory t = 256, horizon h = 5
-        #     # (t h) ... -> h t ...
-        #     batch = batch.reshape(num_slices, horizon).transpose(1, 0).contiguous()
-
-        #     obs = {
-        #         "rgb": batch["observation", "image"][FIRST_FRAME].to(self.device, non_blocking=True),
-        #         "state": batch["observation", "state"][FIRST_FRAME].to(self.device, non_blocking=True),
-        #     }
-        #     action = batch["action"].to(self.device, non_blocking=True)
-        #     next_obses = {
-        #         "rgb": batch["next", "observation", "image"].to(self.device, non_blocking=True),
-        #         "state": batch["next", "observation", "state"].to(self.device, non_blocking=True),
-        #     }
-        #     reward = batch["next", "reward"].to(self.device, non_blocking=True)
-
-        #     idxs = batch["index"][FIRST_FRAME].to(self.device, non_blocking=True)
-        #     weights = batch["_weight"][FIRST_FRAME, :, None].to(self.device, non_blocking=True)
-
-        #     # TODO(rcadene): rearrange directly in offline dataset
-        #     if reward.ndim == 2:
-        #         reward = einops.rearrange(reward, "h t -> h t 1")
-
-        #     assert reward.ndim == 3
-        #     assert reward.shape == (horizon, num_slices, 1)
-        #     # We dont use `batch["next", "done"]` since it only indicates the end of an
-        #     # episode, but not the end of the trajectory of an episode.
-        #     # Neither does `batch["next", "terminated"]`
-        #     done = torch.zeros_like(reward, dtype=torch.bool, device=reward.device)
-        #     mask = torch.ones_like(reward, dtype=torch.bool, device=reward.device)
-        #     return obs, action, next_obses, reward, mask, done, idxs, weights
-
-        # batch = replay_buffer.sample(batch_size) if self.cfg.balanced_sampling else replay_buffer.sample()
-
-        # obs, action, next_obses, reward, mask, done, idxs, weights = process_batch(
-        #     batch, self.cfg.horizon, num_slices
-        # )
-
-        # Sample from demonstration dataset
-        # if demo_batch_size > 0:
-        #     demo_batch = demo_buffer.sample(demo_batch_size)
-        #     (
-        #         demo_obs,
-        #         demo_action,
-        #         demo_next_obses,
-        #         demo_reward,
-        #         demo_mask,
-        #         demo_done,
-        #         demo_idxs,
-        #         demo_weights,
-        #     ) = process_batch(demo_batch, self.cfg.horizon, demo_num_slices)
-
-        #     if isinstance(obs, dict):
-        #         obs = {k: torch.cat([obs[k], demo_obs[k]]) for k in obs}
-        #         next_obses = {k: torch.cat([next_obses[k], demo_next_obses[k]], dim=1) for k in next_obses}
-        #     else:
-        #         obs = torch.cat([obs, demo_obs])
-        #         next_obses = torch.cat([next_obses, demo_next_obses], dim=1)
-        #     action = torch.cat([action, demo_action], dim=1)
-        #     reward = torch.cat([reward, demo_reward], dim=1)
-        #     mask = torch.cat([mask, demo_mask], dim=1)
-        #     done = torch.cat([done, demo_done], dim=1)
-        #     idxs = torch.cat([idxs, demo_idxs])
-        #     weights = torch.cat([weights, demo_weights])
-
         batch_size = batch["index"].shape[0]
 
         # TODO(rcadene): convert tdmpc with (batch size, time/horizon, channels)
@@ -534,6 +446,7 @@ class TDMPCPolicy(nn.Module):
         )
         self.optim.step()
 
+        # TODO(rcadene): implement PrioritizedSampling by modifying sampler.weights with priorities computed by a criterion
         # if self.cfg.per:
         #     # Update priorities
         #     priorities = priority_loss.clamp(max=1e4).detach()
diff --git a/lerobot/configs/env/aloha.yaml b/lerobot/configs/env/aloha.yaml
index 7a8d8b58..6b836795 100644
--- a/lerobot/configs/env/aloha.yaml
+++ b/lerobot/configs/env/aloha.yaml
@@ -18,7 +18,6 @@ env:
   from_pixels: True
   pixels_only: False
   image_size: [3, 480, 640]
-  action_repeat: 1
   episode_length: 400
   fps: ${fps}
 
diff --git a/lerobot/configs/env/pusht.yaml b/lerobot/configs/env/pusht.yaml
index a5fbcc25..a7097ffd 100644
--- a/lerobot/configs/env/pusht.yaml
+++ b/lerobot/configs/env/pusht.yaml
@@ -18,7 +18,6 @@ env:
   from_pixels: True
   pixels_only: False
   image_size: 96
-  action_repeat: 1
   episode_length: 300
   fps: ${fps}
 
diff --git a/lerobot/configs/env/xarm.yaml b/lerobot/configs/env/xarm.yaml
index 8b3c72ef..bcba659e 100644
--- a/lerobot/configs/env/xarm.yaml
+++ b/lerobot/configs/env/xarm.yaml
@@ -17,7 +17,6 @@ env:
   from_pixels: True
   pixels_only: False
   image_size: 84
-  # action_repeat: 2  # we can remove if policy has n_action_steps=2
   episode_length: 25
   fps: ${fps}
 
diff --git a/lerobot/configs/policy/tdmpc.yaml b/lerobot/configs/policy/tdmpc.yaml
index 2ebaad9b..4fd2b6bb 100644
--- a/lerobot/configs/policy/tdmpc.yaml
+++ b/lerobot/configs/policy/tdmpc.yaml
@@ -36,6 +36,7 @@ policy:
   log_std_max: 2
 
   # learning
+  batch_size: 256
   max_buffer_size: 10000
   horizon: 5
   reward_coef: 0.5
diff --git a/lerobot/scripts/eval.py b/lerobot/scripts/eval.py
index 512bb451..394a5d15 100644
--- a/lerobot/scripts/eval.py
+++ b/lerobot/scripts/eval.py
@@ -32,6 +32,7 @@ import json
 import logging
 import threading
 import time
+from copy import deepcopy
 from datetime import datetime as dt
 from pathlib import Path
 
@@ -57,14 +58,14 @@ def write_video(video_path, stacked_frames, fps):
 def eval_policy(
     env: gym.vector.VectorEnv,
     policy,
-    save_video: bool = False,
+    max_episodes_rendered: int = 0,
     video_dir: Path = None,
     # TODO(rcadene): make it possible to overwrite fps? we should use env.fps
-    fps: int = 15,
-    return_first_video: bool = False,
     transform: callable = None,
     seed=None,
 ):
+    fps = env.unwrapped.metadata["render_fps"]
+
     if policy is not None:
         policy.eval()
     device = "cpu" if policy is None else next(policy.parameters()).device
@@ -83,14 +84,11 @@ def eval_policy(
     # needed as I'm currently taking a ceil.
     ep_frames = []
 
-    def maybe_render_frame(env):
-        if save_video:  # noqa: B023
-            if return_first_video:
-                visu = env.envs[0].render()
-                visu = visu[None, ...]  # add batch dim
-            else:
-                visu = np.stack([env.render() for env in env.envs])
-            ep_frames.append(visu)  # noqa: B023
+    def render_frame(env):
+        # noqa: B023
+        eps_rendered = min(max_episodes_rendered, len(env.envs))
+        visu = np.stack([env.envs[i].render() for i in range(eps_rendered)])
+        ep_frames.append(visu)  # noqa: B023
 
     for _ in range(num_episodes):
         seeds.append("TODO")
@@ -104,8 +102,14 @@ def eval_policy(
 
     # reset the environment
     observation, info = env.reset(seed=seed)
-    maybe_render_frame(env)
+    if max_episodes_rendered > 0:
+        render_frame(env)
 
+    observations = []
+    actions = []
+    # episode
+    # frame_id
+    # timestamp
     rewards = []
     successes = []
     dones = []
@@ -113,8 +117,13 @@ def eval_policy(
     done = torch.tensor([False for _ in env.envs])
     step = 0
     while not done.all():
+        # format from env keys to lerobot keys
+        observation = preprocess_observation(observation)
+        observations.append(deepcopy(observation))
+
         # apply transform to normalize the observations
-        observation = preprocess_observation(observation, transform)
+        for key in observation:
+            observation[key] = torch.stack([transform({key: item})[key] for item in observation[key]])
 
         # send observation to device/gpu
         observation = {key: observation[key].to(device, non_blocking=True) for key in observation}
@@ -128,9 +137,11 @@ def eval_policy(
 
         # apply the next
         observation, reward, terminated, truncated, info = env.step(action)
-        maybe_render_frame(env)
+        if max_episodes_rendered > 0:
+            render_frame(env)
 
         # TODO(rcadene): implement a wrapper over env to return torch tensors in float32 (and cuda?)
+        action = torch.from_numpy(action)
         reward = torch.from_numpy(reward)
         terminated = torch.from_numpy(terminated)
         truncated = torch.from_numpy(truncated)
@@ -147,12 +158,24 @@ def eval_policy(
             success = [False for _ in env.envs]
         success = torch.tensor(success)
 
+        actions.append(action)
         rewards.append(reward)
         dones.append(done)
         successes.append(success)
 
         step += 1
 
+    env.close()
+
+    # add the last observation when the env is done
+    observation = preprocess_observation(observation)
+    observations.append(deepcopy(observation))
+
+    new_obses = {}
+    for key in observations[0].keys():  # noqa: SIM118
+        new_obses[key] = torch.stack([obs[key] for obs in observations], dim=1)
+    observations = new_obses
+    actions = torch.stack(actions, dim=1)
     rewards = torch.stack(rewards, dim=1)
     successes = torch.stack(successes, dim=1)
     dones = torch.stack(dones, dim=1)
@@ -172,29 +195,61 @@ def eval_policy(
     max_rewards.extend(batch_max_reward.tolist())
     all_successes.extend(batch_success.tolist())
 
-    env.close()
+    # similar logic is implemented in dataset preprocessing
+    ep_dicts = []
+    num_episodes = dones.shape[0]
+    total_frames = 0
+    idx0 = idx1 = 0
+    data_ids_per_episode = {}
+    for ep_id in range(num_episodes):
+        num_frames = done_indices[ep_id].item() + 1
+        # TODO(rcadene): We need to add a missing last frame which is the observation
+        # of a done state. it is critical to have this frame for tdmpc to predict a "done observation/state"
+        ep_dict = {
+            "action": actions[ep_id, :num_frames],
+            "episode": torch.tensor([ep_id] * num_frames),
+            "frame_id": torch.arange(0, num_frames, 1),
+            "timestamp": torch.arange(0, num_frames, 1) / fps,
+            "next.done": dones[ep_id, :num_frames],
+            "next.reward": rewards[ep_id, :num_frames].type(torch.float32),
+        }
+        for key in observations:
+            ep_dict[key] = observations[key][ep_id, :num_frames]
+        ep_dicts.append(ep_dict)
 
-    if save_video or return_first_video:
+        total_frames += num_frames
+        idx1 += num_frames
+
+        data_ids_per_episode[ep_id] = torch.arange(idx0, idx1, 1)
+
+        idx0 = idx1
+
+    # similar logic is implemented in dataset preprocessing
+    data_dict = {}
+    keys = ep_dicts[0].keys()
+    for key in keys:
+        data_dict[key] = torch.cat([x[key] for x in ep_dicts])
+    data_dict["index"] = torch.arange(0, total_frames, 1)
+
+    if max_episodes_rendered > 0:
         batch_stacked_frames = np.stack(ep_frames, 1)  # (b, t, *)
 
-        if save_video:
-            for stacked_frames, done_index in zip(
-                batch_stacked_frames, done_indices.flatten().tolist(), strict=False
-            ):
-                if episode_counter >= num_episodes:
-                    continue
-                video_dir.mkdir(parents=True, exist_ok=True)
-                video_path = video_dir / f"eval_episode_{episode_counter}.mp4"
-                thread = threading.Thread(
-                    target=write_video,
-                    args=(str(video_path), stacked_frames[:done_index], fps),
-                )
-                thread.start()
-                threads.append(thread)
-                episode_counter += 1
+        for stacked_frames, done_index in zip(
+            batch_stacked_frames, done_indices.flatten().tolist(), strict=False
+        ):
+            if episode_counter >= num_episodes:
+                continue
+            video_dir.mkdir(parents=True, exist_ok=True)
+            video_path = video_dir / f"eval_episode_{episode_counter}.mp4"
+            thread = threading.Thread(
+                target=write_video,
+                args=(str(video_path), stacked_frames[:done_index], fps),
+            )
+            thread.start()
+            threads.append(thread)
+            episode_counter += 1
 
-        if return_first_video:
-            first_video = batch_stacked_frames[0].transpose(0, 3, 1, 2)
+        videos = batch_stacked_frames.transpose(0, 3, 1, 2)
 
     for thread in threads:
         thread.join()
@@ -225,9 +280,13 @@ def eval_policy(
             "eval_s": time.time() - start,
             "eval_ep_s": (time.time() - start) / num_episodes,
         },
+        "episodes": {
+            "data_dict": data_dict,
+            "data_ids_per_episode": data_ids_per_episode,
+        },
     }
-    if return_first_video:
-        return info, first_video
+    if max_episodes_rendered > 0:
+        info["videos"] = videos
     return info
 
 
@@ -259,7 +318,7 @@ def eval(cfg: dict, out_dir=None, stats_path=None):
     info = eval_policy(
         env,
         policy=policy,
-        save_video=True,
+        max_episodes_rendered=10,
         video_dir=Path(out_dir) / "eval",
         fps=cfg.env.fps,
         # TODO(rcadene): what should we do with the transform?
diff --git a/lerobot/scripts/train.py b/lerobot/scripts/train.py
index cca26902..6dfbd12b 100644
--- a/lerobot/scripts/train.py
+++ b/lerobot/scripts/train.py
@@ -1,8 +1,8 @@
 import logging
+from copy import deepcopy
 from pathlib import Path
 
 import hydra
-import numpy as np
 import torch
 
 from lerobot.common.datasets.factory import make_dataset
@@ -110,6 +110,64 @@ def log_eval_info(logger, info, step, cfg, dataset, is_offline):
     logger.log_dict(info, step, mode="eval")
 
 
+def calculate_online_sample_weight(n_off: int, n_on: int, pc_on: float):
+    """
+    Calculate the sampling weight to be assigned to samples so that a specified percentage of the batch comes from online dataset (on average).
+
+    Parameters:
+    - n_off (int): Number of offline samples, each with a sampling weight of 1.
+    - n_on (int): Number of online samples.
+    - pc_on (float): Desired percentage of online samples in decimal form (e.g., 50% as 0.5).
+
+    The total weight of offline samples is n_off * 1.0.
+    The total weight of offline samples is n_on * w.
+    The total combined weight of all samples is n_off + n_on * w.
+    The fraction of the weight that is online is n_on * w / (n_off + n_on * w).
+    We want this fraction to equal pc_on, so we set up the equation n_on * w / (n_off + n_on * w) = pc_on.
+    The solution is w = - (n_off * pc_on) / (n_on * (pc_on - 1))
+    """
+    assert 0.0 <= pc_on <= 1.0
+    return -(n_off * pc_on) / (n_on * (pc_on - 1))
+
+
+def add_episodes_inplace(episodes, online_dataset, concat_dataset, sampler, pc_online_samples):
+    data_dict = episodes["data_dict"]
+    data_ids_per_episode = episodes["data_ids_per_episode"]
+
+    if len(online_dataset) == 0:
+        # initialize online dataset
+        online_dataset.data_dict = data_dict
+        online_dataset.data_ids_per_episode = data_ids_per_episode
+    else:
+        # find episode index and data frame indices according to previous episode in online_dataset
+        start_episode = max(online_dataset.data_ids_per_episode.keys()) + 1
+        start_index = online_dataset.data_dict["index"][-1].item() + 1
+        data_dict["episode"] += start_episode
+        data_dict["index"] += start_index
+
+        # extend online dataset
+        for key in data_dict:
+            # TODO(rcadene): avoid reallocating memory at every step by preallocating memory or changing our data structure
+            online_dataset.data_dict[key] = torch.cat([online_dataset.data_dict[key], data_dict[key]])
+        for ep_id in data_ids_per_episode:
+            online_dataset.data_ids_per_episode[ep_id + start_episode] = (
+                data_ids_per_episode[ep_id] + start_index
+            )
+
+    # update the concatenated dataset length used during sampling
+    concat_dataset.cumulative_sizes = concat_dataset.cumsum(concat_dataset.datasets)
+
+    # update the sampling weights for each frame so that online frames get sampled a certain percentage of times
+    len_online = len(online_dataset)
+    len_offline = len(concat_dataset) - len_online
+    weight_offline = 1.0
+    weight_online = calculate_online_sample_weight(len_offline, len_online, pc_online_samples)
+    sampler.weights = torch.tensor([weight_offline] * len_offline + [weight_online] * len(online_dataset))
+
+    # update the total number of samples used during sampling
+    sampler.num_samples = len(concat_dataset)
+
+
 def train(cfg: dict, out_dir=None, job_name=None):
     if out_dir is None:
         raise NotImplementedError()
@@ -128,26 +186,7 @@ def train(cfg: dict, out_dir=None, job_name=None):
     set_global_seed(cfg.seed)
 
     logging.info("make_dataset")
-    dataset = make_dataset(cfg)
-
-    # TODO(rcadene): move balanced_sampling, per_alpha, per_beta outside policy
-    # if cfg.policy.balanced_sampling:
-    #     logging.info("make online_buffer")
-    #     num_traj_per_batch = cfg.policy.batch_size
-
-    #     online_sampler = PrioritizedSliceSampler(
-    #         max_capacity=100_000,
-    #         alpha=cfg.policy.per_alpha,
-    #         beta=cfg.policy.per_beta,
-    #         num_slices=num_traj_per_batch,
-    #         strict_length=True,
-    #     )
-
-    #     online_buffer = TensorDictReplayBuffer(
-    #         storage=LazyMemmapStorage(100_000),
-    #         sampler=online_sampler,
-    #         transform=dataset.transform,
-    #     )
+    offline_dataset = make_dataset(cfg)
 
     logging.info("make_env")
     env = make_env(cfg, num_parallel_envs=cfg.eval_episodes)
@@ -165,9 +204,8 @@ def train(cfg: dict, out_dir=None, job_name=None):
     logging.info(f"{cfg.env.task=}")
     logging.info(f"{cfg.offline_steps=} ({format_big_number(cfg.offline_steps)})")
     logging.info(f"{cfg.online_steps=}")
-    logging.info(f"{cfg.env.action_repeat=}")
-    logging.info(f"{dataset.num_samples=} ({format_big_number(dataset.num_samples)})")
-    logging.info(f"{dataset.num_episodes=}")
+    logging.info(f"{offline_dataset.num_samples=} ({format_big_number(offline_dataset.num_samples)})")
+    logging.info(f"{offline_dataset.num_episodes=}")
     logging.info(f"{num_learnable_params=} ({format_big_number(num_learnable_params)})")
     logging.info(f"{num_total_params=} ({format_big_number(num_total_params)})")
 
@@ -175,18 +213,17 @@ def train(cfg: dict, out_dir=None, job_name=None):
     def _maybe_eval_and_maybe_save(step):
         if step % cfg.eval_freq == 0:
             logging.info(f"Eval policy at step {step}")
-            eval_info, first_video = eval_policy(
+            eval_info = eval_policy(
                 env,
                 policy,
-                return_first_video=True,
                 video_dir=Path(out_dir) / "eval",
-                save_video=True,
-                transform=dataset.transform,
+                max_episodes_rendered=4,
+                transform=offline_dataset.transform,
                 seed=cfg.seed,
             )
-            log_eval_info(logger, eval_info["aggregated"], step, cfg, dataset, is_offline)
+            log_eval_info(logger, eval_info["aggregated"], step, cfg, offline_dataset, is_offline)
             if cfg.wandb.enable:
-                logger.log_video(first_video, step, mode="eval")
+                logger.log_video(eval_info["videos"][0], step, mode="eval")
             logging.info("Resume training")
 
         if cfg.save_model and step % cfg.save_freq == 0:
@@ -194,11 +231,9 @@ def train(cfg: dict, out_dir=None, job_name=None):
             logger.save_model(policy, identifier=step)
             logging.info("Resume training")
 
-    step = 0  # number of policy update (forward + backward + optim)
-
-    is_offline = True
+    # create dataloader for offline training
     dataloader = torch.utils.data.DataLoader(
-        dataset,
+        offline_dataset,
         num_workers=4,
         batch_size=cfg.policy.batch_size,
         shuffle=True,
@@ -206,6 +241,9 @@ def train(cfg: dict, out_dir=None, job_name=None):
         drop_last=True,
     )
     dl_iter = cycle(dataloader)
+
+    step = 0  # number of policy update (forward + backward + optim)
+    is_offline = True
     for offline_step in range(cfg.offline_steps):
         if offline_step == 0:
             logging.info("Start offline training on a fixed dataset")
@@ -219,7 +257,7 @@ def train(cfg: dict, out_dir=None, job_name=None):
 
         # TODO(rcadene): is it ok if step_t=0 = 0 and not 1 as previously done?
         if step % cfg.log_freq == 0:
-            log_train_info(logger, train_info, step, cfg, dataset, is_offline)
+            log_train_info(logger, train_info, step, cfg, offline_dataset, is_offline)
 
         # Note: _maybe_eval_and_maybe_save happens **after** the `step`th training update has completed, so we pass in
         # step + 1.
@@ -227,61 +265,60 @@ def train(cfg: dict, out_dir=None, job_name=None):
 
         step += 1
 
-    raise NotImplementedError()
+    # create an env dedicated to online episodes collection from policy rollout
+    rollout_env = make_env(cfg, num_parallel_envs=1)
+
+    # create an empty online dataset similar to offline dataset
+    online_dataset = deepcopy(offline_dataset)
+    online_dataset.data_dict = {}
+    online_dataset.data_ids_per_episode = {}
+
+    # create dataloader for online training
+    concat_dataset = torch.utils.data.ConcatDataset([offline_dataset, online_dataset])
+    weights = [1.0] * len(concat_dataset)
+    sampler = torch.utils.data.WeightedRandomSampler(
+        weights, num_samples=len(concat_dataset), replacement=True
+    )
+    dataloader = torch.utils.data.DataLoader(
+        concat_dataset,
+        num_workers=4,
+        batch_size=cfg.policy.batch_size,
+        sampler=sampler,
+        pin_memory=cfg.device != "cpu",
+        drop_last=True,
+    )
+    dl_iter = cycle(dataloader)
 
-    demo_buffer = dataset if cfg.policy.balanced_sampling else None
     online_step = 0
     is_offline = False
     for env_step in range(cfg.online_steps):
         if env_step == 0:
             logging.info("Start online training by interacting with environment")
-        # TODO: add configurable number of rollout? (default=1)
+
         with torch.no_grad():
-            rollout = env.rollout(
-                max_steps=cfg.env.episode_length,
-                policy=policy,
-                auto_cast_to_device=True,
+            eval_info = eval_policy(
+                rollout_env,
+                policy,
+                transform=offline_dataset.transform,
+                seed=cfg.seed,
             )
 
-        assert (
-            len(rollout.batch_size) == 2
-        ), "2 dimensions expected: number of env in parallel x max number of steps during rollout"
-
-        num_parallel_env = rollout.batch_size[0]
-        if num_parallel_env != 1:
-            # TODO(rcadene): when num_parallel_env > 1, rollout["episode"] needs to be properly set and we need to add tests
-            raise NotImplementedError()
-
-        num_max_steps = rollout.batch_size[1]
-        assert num_max_steps <= cfg.env.episode_length
-
-        # reshape to have a list of steps to insert into online_buffer
-        rollout = rollout.reshape(num_parallel_env * num_max_steps)
-
-        # set same episode index for all time steps contained in this rollout
-        rollout["episode"] = torch.tensor([env_step] * len(rollout), dtype=torch.int)
-        # online_buffer.extend(rollout)
-
-        ep_sum_reward = rollout["next", "reward"].sum()
-        ep_max_reward = rollout["next", "reward"].max()
-        ep_success = rollout["next", "success"].any()
-        rollout_info = {
-            "avg_sum_reward": np.nanmean(ep_sum_reward),
-            "avg_max_reward": np.nanmean(ep_max_reward),
-            "pc_success": np.nanmean(ep_success) * 100,
-            "env_step": env_step,
-            "ep_length": len(rollout),
-        }
+            online_pc_sampling = cfg.get("demo_schedule", 0.5)
+            add_episodes_inplace(
+                eval_info["episodes"], online_dataset, concat_dataset, sampler, online_pc_sampling
+            )
 
         for _ in range(cfg.policy.utd):
-            train_info = policy.update(
-                # online_buffer,
-                step,
-                demo_buffer=demo_buffer,
-            )
+            policy.train()
+            batch = next(dl_iter)
+
+            for key in batch:
+                batch[key] = batch[key].to(cfg.device, non_blocking=True)
+
+            train_info = policy(batch, step)
+
             if step % cfg.log_freq == 0:
-                train_info.update(rollout_info)
-                log_train_info(logger, train_info, step, cfg, dataset, is_offline)
+                log_train_info(logger, train_info, step, cfg, online_dataset, is_offline)
 
             # Note: _maybe_eval_and_maybe_save happens **after** the `step`th training update has completed, so we pass
             # in step + 1.