Raise error when `topk>1` and `page>1` for paged attention backends. (#9784)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

python/sglang/srt/server_args.py

@@ -747,6 +747,15 @@ class ServerArgs:
                 )
                 self.speculative_num_draft_tokens = self.speculative_num_steps + 1
 
+            if (
+                self.speculative_eagle_topk > 1
+                and self.page_size > 1
+                and self.attention_backend != "flashinfer"
+            ):
+                raise ValueError(
+                    "speculative_eagle_topk > 1 with page_size > 1 is unstable and produces incorrect results for paged attention backends. This combination is only supported for the 'flashinfer' backend."
+                )
+
             # The token generated from the verify step is counted.
             # If sepculative_num_steps >= speculative_num_draft_tokens, the additional tokens will definitely be discarded.
             # assert self.speculative_num_steps < self.speculative_num_draft_tokens