Enable logging all the information returned by the forward methods of policies (#151)

lerobot/common/policies/policy_protocol.py

@@ -38,7 +38,8 @@ class Policy(Protocol):
     def forward(self, batch: dict[str, Tensor]) -> dict:
         """Run the batch through the model and compute the loss for training or validation.
 
-        Returns a dictionary with "loss" and maybe other information.
+        Returns a dictionary with "loss" and potentially other information. Apart from "loss" which is a Tensor, all
+        other items should be logging-friendly, native Python types.
         """
 
     def select_action(self, batch: dict[str, Tensor]):