Creating a LowerTriangularMask no longer creates a CUDA tensor (fairinternal/xformers#1274)

__original_commit__ = fairinternal/xformers@4a6a2a1

Author: danthe3rd

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [0.0.28.post3] - TBD
+### Fixed:
+- Creating a `LowerTriangularMask` no longer creates a CUDA tensor
 ### Removed:
 - Following PyTorch, xFormers no longer builds binaries for conda. Pip is now the only recommended way to get xFormers
 

xformers/ops/fmha/attn_bias.py

@@ -1589,18 +1589,8 @@ class AttentionBiasSubTensor(torch.Tensor, AttentionBias):
     _subtensor: torch.Tensor
 
     @staticmethod
-    def __new__(cls, *, _subtensor=None):
-        if _subtensor is None:
-            _subtensor = torch.empty((0,), device=_get_default_bias_device())
-        tensor = torch.Tensor._make_wrapper_subclass(  # type: ignore[attr-defined]
-            cls,
-            [],
-            device=_subtensor.device,
-            dtype=_subtensor.dtype,
-            requires_grad=False,
-        )
-        tensor._subtensor = _subtensor
-        return tensor
+    def __new__(cls, *, _subtensor=None, device=None, **kwargs):
+        raise NotImplementedError()
 
     def __init__(self, *args, **kwargs) -> None:
         super().__init__()
@@ -1667,6 +1657,24 @@ class LowerTriangularMask(AttentionBiasSubTensor):
 
     HOLDS_DENSE_TENSOR = False
 
+    @staticmethod
+    def __new__(cls, *, _subtensor=None, device="cpu", **kwargs):
+        """
+        Note: create on CPU by default to avoid initializing CUDA context
+        by mistake.
+        """
+        if _subtensor is None:
+            _subtensor = torch.empty((0,), device=device)
+        tensor = torch.Tensor._make_wrapper_subclass(  # type: ignore[attr-defined]
+            cls,
+            [],
+            device=_subtensor.device,
+            dtype=_subtensor.dtype,
+            requires_grad=False,
+        )
+        tensor._subtensor = _subtensor
+        return tensor
+
     def materialize(
         self,
         shape: Tuple[int, ...],