Optimize GPU memory usage in FlashAttentionBackend's strided indexing (#5262)

Co-authored-by: ch-wan <cwan39@gatech.edu>

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -977,10 +977,12 @@ class FlashAttentionBackend(AttentionBackend):
                     metadata.max_seq_len_k + self.page_size - 1
                 ) // self.page_size
                 page_indices = self.req_to_token[
-                    :,
-                    self.decode_cuda_graph_metadata["strided_indices"][:max_seq_pages],
+                    req_pool_indices[:, None],
+                    self.decode_cuda_graph_metadata["strided_indices"][:max_seq_pages][
+                        None, :
+                    ],
                 ]
-                page_indices = page_indices[req_pool_indices] // self.page_size
+                page_indices //= self.page_size
                 metadata.page_table[:, :max_seq_pages].copy_(page_indices)
                 metadata.page_table[:, max_seq_pages:].fill_(0)