Fix: Prevent Invalid next_state References When optimize_memory=True (#918)

lerobot/scripts/server/buffer.py

@@ -286,9 +286,10 @@ class ReplayBuffer:
             raise RuntimeError("Cannot sample from an empty buffer. Add transitions first.")
 
         batch_size = min(batch_size, self.size)
+        high = max(0, self.size - 1) if self.optimize_memory and self.size < self.capacity else self.size
 
         # Random indices for sampling - create on the same device as storage
-        idx = torch.randint(low=0, high=self.size, size=(batch_size,), device=self.storage_device)
+        idx = torch.randint(low=0, high=high, size=(batch_size,), device=self.storage_device)
 
         # Identify image keys that need augmentation
         image_keys = [k for k in self.states if k.startswith("observation.image")] if self.use_drq else []