Add tree attention backend for v1 (part 1) (#20401)

Signed-off-by: Giancarlo Delfin <gdelfin@meta.com>

vllm/v1/attention/backends/utils.py

@@ -214,6 +214,26 @@ class AttentionMetadataBuilder(abc.ABC, Generic[M]):
         return self.build(common_prefix_len=0,
                           common_attn_metadata=common_attn_metadata)
 
+    def build_for_drafting(
+        self,
+        common_attn_metadata: CommonAttentionMetadata,
+        draft_index: int,
+    ) -> M:
+        """
+        Build attention metadata for draft model. Uses build by default.
+        
+        Args:
+            common_attn_metadata: The common attention metadata.
+            draft_index: The index of the current draft operation.
+                When speculating a chain of tokens, this index refers to the
+                draft attempt for the i-th token.
+                For tree-based attention, this index instead refers to the
+                draft attempt for the i-th level in the tree of tokens.
+        """
+        return self.build(common_prefix_len=0,
+                          common_attn_metadata=common_attn_metadata,
+                          fast_build=True)
+
     def use_cascade_attention(
         self,
         common_prefix_len: int,