[Executorch][llama] Allow custom sdpa op replacement pass to leverage attention mask

Pull Request resolved: #10285

Previously we assumed that the custom sdpa always does causal attention.
This diff adds option to this module swap pass to make custom sdpa leverage
attention mask instead of causal.
ghstack-source-id: 278977227
@exported-using-ghexport

Differential Revision: [D73222736](https://our.internmc.facebook.com/intern/diff/D73222736/)

Author: kimishpatel

examples/models/llama/source_transformation/sdpa.py

@@ -22,9 +22,15 @@ class SDPACustom(torch.nn.Module):
     def __init__(
         self,
         dim: int,
+        max_context_len,
+        enable_dynamic_shape,
+        use_attention_mask: bool = False,
     ):
         super().__init__()
         self.dim = dim
+        self.max_context_len = max_context_len
+        self.use_attention_mask = use_attention_mask
+        self.enable_dynamic_shape = enable_dynamic_shape
 
     def forward(
         self,
@@ -36,6 +42,15 @@ def forward(
         seqlen,
         mask,
     ):
+        if self.enable_dynamic_shape:
+            start_pos = input_pos[-1].item()
+            torch._check_is_size(start_pos)
+            torch._check(start_pos < self.max_context_len)
+            seq_length = q.size(2)
+            mask = mask.narrow(0, start_pos, seq_length)
+        else:
+            mask = mask[input_pos]
+
         q = q.transpose(1, 2)  # (bs, seqlen, n_local_heads, head_dim)
         k = k.transpose(1, 2)
         v = v.transpose(1, 2)
@@ -47,34 +62,54 @@ def forward(
         k = k.to(dtype=torch.float)
         v = v.to(dtype=torch.float)
 
-        output = torch.ops.llama.custom_sdpa(
-            q,
-            k,
-            v,
-            input_pos[0].item(),
-            None,  # Attention mask
-            0,  # dropout probability. Ignored by the code
-            True,  # is_causal
-        )
+        if self.use_attention_mask:
+            output = torch.ops.llama.custom_sdpa(
+                q,
+                k,
+                v,
+                input_pos[0].item(),
+                mask,  # Attention mask
+                0,  # dropout probability. Ignored by the code
+                False,  # is_causal
+            )
+        else:
+            output = torch.ops.llama.custom_sdpa(
+                q,
+                k,
+                v,
+                input_pos[0].item(),
+                None,  # Attention mask
+                0,  # dropout probability. Ignored by the code
+                True,  # is_causal
+            )
         return output.view(bsz, seqlen, self.dim).to(dtype=input_dtype)
 
 
-def _replace_sdpa_with_custom_op(module: torch.nn.Module):
+def _replace_sdpa_with_custom_op(
+    module: torch.nn.Module, use_attention_mask: bool = False
+):
     for name, child in module.named_children():
         if isinstance(child, SDPA):
             setattr(
                 module,
                 name,
-                SDPACustom(child.dim),
+                SDPACustom(
+                    child.dim,
+                    child.max_context_len,
+                    child.enable_dynamic_shape,
+                    use_attention_mask=use_attention_mask,
+                ),
             )
         else:
-            _replace_sdpa_with_custom_op(child)
+            _replace_sdpa_with_custom_op(child, use_attention_mask=use_attention_mask)
 
 
-def replace_sdpa_with_custom_op(module: torch.nn.Module) -> torch.nn.Module:
+def replace_sdpa_with_custom_op(
+    module: torch.nn.Module, use_attention_mask: bool = False
+) -> torch.nn.Module:
     from executorch.extension.llm.custom_ops import custom_ops  # noqa
 
-    _replace_sdpa_with_custom_op(module)
+    _replace_sdpa_with_custom_op(module, use_attention_mask=use_attention_mask)
     return module