[BugFix] Propagate maybe_dense_stack in _stack

ghstack-source-id: a1cb1de
Pull Request resolved: #1036

Author: Vincent Moens

tensordict/_torch_func.py

@@ -414,10 +414,12 @@ def _stack(
     out: T | None = None,
     strict: bool = False,
     contiguous: bool = False,
-    maybe_dense_stack: bool = False,
+    maybe_dense_stack: bool | None = None,
 ) -> T:
     if not len(list_of_tensordicts):
         raise RuntimeError("list_of_tensordicts cannot be empty")
+    if maybe_dense_stack is None:
+        maybe_dense_stack = lazy_legacy()
     is_tc = any(is_tensorclass(td) for td in list_of_tensordicts)
     if all(is_non_tensor(td) for td in list_of_tensordicts):
         from tensordict.tensorclass import NonTensorData
@@ -457,7 +459,11 @@ def _stack(
                 if not _lazy_legacy and not contiguous:
                     if maybe_dense_stack:
                         with set_lazy_legacy(True):
-                            return _stack(list_of_tensordicts, dim=dim)
+                            return _stack(
+                                list_of_tensordicts,
+                                dim=dim,
+                                maybe_dense_stack=maybe_dense_stack,
+                            )
                     else:
                         raise RuntimeError(
                             "The sets of keys in the tensordicts to stack are exclusive. "
@@ -490,7 +496,11 @@ def _stack(
                             dim = dim - 1
                         return LazyStackedTensorDict(
                             *[
-                                _stack(list(subtds), dim=dim)
+                                _stack(
+                                    list(subtds),
+                                    dim=dim,
+                                    maybe_dense_stack=maybe_dense_stack,
+                                )
                                 for subtds in _zip_strict(
                                     *[td.tensordicts for td in list_of_tensordicts]
                                 )
@@ -540,7 +550,11 @@ def _stack(
                                 # Nested tensors will require a lazy stack
                                 if maybe_dense_stack:
                                     with set_lazy_legacy(True):
-                                        return _stack(list_of_tensordicts, dim=dim)
+                                        return _stack(
+                                            list_of_tensordicts,
+                                            dim=dim,
+                                            maybe_dense_stack=maybe_dense_stack,
+                                        )
                                 else:
                                     raise RuntimeError(
                                         f"The shapes of the tensors to stack is incompatible: {new_tensor_shape} vs {tensor_shape} for key {key}."