Remove offline training, refactor train.py and logging/checkpointing (#670)

Co-authored-by: Remi <remi.cadene@huggingface.co>

lerobot/common/policies/diffusion/modeling_diffusion.py

@@ -143,7 +143,7 @@ class DiffusionPolicy(PreTrainedPolicy):
         action = self._queues["action"].popleft()
         return action
 
-    def forward(self, batch: dict[str, Tensor]) -> dict[str, Tensor]:
+    def forward(self, batch: dict[str, Tensor]) -> tuple[Tensor, None]:
         """Run the batch through the model and compute the loss for training or validation."""
         batch = self.normalize_inputs(batch)
         if self.config.image_features:
@@ -153,7 +153,8 @@ class DiffusionPolicy(PreTrainedPolicy):
             )
         batch = self.normalize_targets(batch)
         loss = self.diffusion.compute_loss(batch)
-        return {"loss": loss}
+        # no output_dict so returning None
+        return loss, None
 
 
 def _make_noise_scheduler(name: str, **kwargs: dict) -> DDPMScheduler | DDIMScheduler: