[BugFix] Fix ModelBasedEnvBase for torch.compile compatibility

Convert spec keys to list before unpacking, as dynamo cannot unpack
_CompositeSpecKeysView objects during tracing.


ghstack-source-id: 959b1f3
Pull-Request: #3299

Author: vmoens

torchrl/envs/model_based/common.py

@@ -161,12 +161,13 @@ def _step(
         else:
             tensordict_out = self.world_model(tensordict_out)
         # done can be missing, it will be filled by `step`
-        tensordict_out = tensordict_out.select(
-            *self.observation_spec.keys(),
-            *self.full_done_spec.keys(),
-            *self.full_reward_spec.keys(),
-            strict=False,
+        # Convert to list for torch.compile compatibility (dynamo can't unpack _CompositeSpecKeysView)
+        keys_to_select = (
+            list(self.observation_spec.keys())
+            + list(self.full_done_spec.keys())
+            + list(self.full_reward_spec.keys())
         )
+        tensordict_out = tensordict_out.select(*keys_to_select, strict=False)
         return tensordict_out
 
     @abc.abstractmethod