Initial commit

fbshipit-source-id: da6be2f26e3a1202f4bffde8cb980e2dcb851294

sam3/perflib/fa3.py

@@ -0,0 +1,27 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates. All Rights Reserved
+
+import torch
+
+
+@torch.library.custom_op("flash::flash_attn_func", mutates_args=())
+def flash_attn_func_op(
+    q: torch.Tensor, k: torch.Tensor, v: torch.Tensor
+) -> torch.Tensor:
+    from flash_attn_interface import flash_attn_func as fa3
+
+    return fa3(q, k, v)
+
+
+def flash_attn_func(q, k, v):
+    dtype = torch.float8_e4m3fn
+    return flash_attn_func_op(q.to(dtype), k.to(dtype), v.to(dtype)).to(q.dtype)
+
+
+@flash_attn_func_op.register_fake
+def _(q, k, v, **kwargs):
+    # two outputs:
+    # 1. output: (batch, seq_len, num_heads, head_dim)
+    # 2. softmax_lse: (batch, num_heads, seq_len) with dtype=torch.float32
+    # output needs to be bfloat16, not float8!
+    meta_q = torch.empty_like(q, dtype=torch.bfloat16).contiguous()
+    return meta_q