Enhance training information logging in learner server

- Added tracking for replay buffer size and offline replay buffer size during training steps.

lerobot/scripts/server/learner_server.py

@@ -681,6 +681,11 @@ def add_actor_information_and_train(
 
         policy.update_target_networks()
         if optimization_step % cfg.training.log_freq == 0:
+            training_infos["replay_buffer_size"] = len(replay_buffer)
+            if offline_replay_buffer is not None:
+                training_infos["offline_replay_buffer_size"] = len(
+                    offline_replay_buffer
+                )
             training_infos["Optimization step"] = optimization_step
             logger.log_dict(
                 d=training_infos, mode="train", custom_step_key="Optimization step"