[Minor] Add env.shape attribute (#1938)

torchrl/envs/common.py

@@ -419,6 +419,13 @@ def run_type_checks(self, run_type_checks: bool) -> None:
 
     @property
     def batch_size(self) -> torch.Size:
+        """Number of envs batched in this environment instance organised in a `torch.Size()` object.
+
+        Environment may be similar or different but it is assumed that they have little if
+        not no interactions between them (e.g., multi-task or batched execution
+        in parallel).
+
+        """
         _batch_size = self.__dict__["_batch_size"]
         if _batch_size is None:
             _batch_size = self._batch_size = torch.Size([])
@@ -439,6 +446,11 @@ def batch_size(self, value: torch.Size) -> None:
             self.input_spec.shape = value
             self.input_spec.lock_()
 
+    @property
+    def shape(self):
+        """Equivalent to :attr:`~.batch_size`."""
+        return self.batch_size
+
     @property
     def device(self) -> torch.device:
         device = self.__dict__.get("_device", None)
@@ -2162,7 +2174,7 @@ def _assert_tensordict_shape(self, tensordict: TensorDictBase) -> None:
             self.batch_locked or self.batch_size != ()
         ) and tensordict.batch_size != self.batch_size:
             raise RuntimeError(
-                f"Expected a tensordict with shape==env.shape, "
+                f"Expected a tensordict with shape==env.batch_size, "
                 f"got {tensordict.batch_size} and {self.batch_size}"
             )