allow flex-attention to be disabled (#19)

* allow flex-attention to silently fail

* allow flex-attn to be disabled via an env var

bytelatent/base_transformer.py

@@ -1,5 +1,5 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
-
+import os
 from enum import Enum
 from typing import Optional, Tuple, Union
 
@@ -16,7 +16,10 @@ from xformers.ops import AttentionBias, fmha
 
 from bytelatent import probe
 
-flex_attention_comp = torch.compile(flex_attention)
+if int(os.environ.get("BLT_ALLOW_MISSING_FLEX_ATTENTION", False)) == 0:
+    flex_attention_comp = torch.compile(flex_attention)
+else:
+    flex_attention_comp = None
 
 
 class InitStdFactor(Enum):

bytelatent/distributed.py

@@ -1,5 +1,4 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
-
 import atexit
 import contextlib
 import logging
@@ -48,9 +47,13 @@ default_no_recompute_ops = {
     torch.ops.aten._scaled_dot_product_flash_attention.default,
     torch.ops.c10d_functional.reduce_scatter_tensor.default,
     torch.ops.xformers_flash.flash_fwd.default,
-    torch.ops.xformers.efficient_attention_forward_cutlass.default,
 }
 
+if int(os.environ.get("BLT_ALLOW_MISSING_FLEX_ATTENTION", False)) == 0:
+    default_no_recompute_ops.add(
+        torch.ops.xformers.efficient_attention_forward_cutlass.default
+    )
+
 
 class DistributedArgs(BaseModel):
     model_config = ConfigDict(extra="forbid")