[Bugfix] Fix missing seq_start_loc in xformers prefill metadata (vllm-project#12464)

Signed-off-by: Isotr0py <2037008807@qq.com>

Author: Isotr0py

vllm/attention/backends/xformers.py

@@ -199,6 +199,8 @@ def prefill_metadata(self) -> Optional["XFormersMetadata"]:
         # Compute some attn_metadata fields which default to None
         query_start_loc = (None if self.query_start_loc is None else
                            self.query_start_loc[:self.num_prefills + 1])
+        seq_start_loc = (None if self.seq_start_loc is None else
+                         self.seq_start_loc[:self.num_prefills + 1])
         slot_mapping = (None if self.slot_mapping is None else
                         self.slot_mapping[:self.num_prefill_tokens])
         seq_lens = (None if self.seq_lens is None else
@@ -225,6 +227,7 @@ def prefill_metadata(self) -> Optional["XFormersMetadata"]:
             max_prefill_seq_len=self.max_prefill_seq_len,
             max_decode_seq_len=0,
             query_start_loc=query_start_loc,
+            seq_start_loc=seq_start_loc,
             context_lens_tensor=context_lens_tensor,
             block_tables=block_tables,
             use_cuda_graph=False,