Make sure policies don't mutate the batch (#323)

lerobot/common/policies/diffusion/modeling_diffusion.py

@@ -122,6 +122,7 @@ class DiffusionPolicy(nn.Module, PyTorchModelHubMixin):
         """
         batch = self.normalize_inputs(batch)
         if len(self.expected_image_keys) > 0:
+            batch = dict(batch)  # shallow copy so that adding a key doesn't modify the original
             batch["observation.images"] = torch.stack([batch[k] for k in self.expected_image_keys], dim=-4)
         # Note: It's important that this happens after stacking the images into a single key.
         self._queues = populate_queues(self._queues, batch)
@@ -143,6 +144,7 @@ class DiffusionPolicy(nn.Module, PyTorchModelHubMixin):
         """Run the batch through the model and compute the loss for training or validation."""
         batch = self.normalize_inputs(batch)
         if len(self.expected_image_keys) > 0:
+            batch = dict(batch)  # shallow copy so that adding a key doesn't modify the original
             batch["observation.images"] = torch.stack([batch[k] for k in self.expected_image_keys], dim=-4)
         batch = self.normalize_targets(batch)
         loss = self.diffusion.compute_loss(batch)