Fix FlashInfer GPU <-> CPU sync (#9409)

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

@@ -1372,6 +1372,13 @@ def fast_decode_plan(
 
         if self.use_tensor_cores:
             # ALSO convert last_page_len to CPU
+            if page_size == 1:
+                # When page size is 1, last_page_len is always 1.
+                # Directly construct the host tensor rather than executing a device-to-host copy.
+                last_page_len_host = torch.ones(
+                    (batch_size,), dtype=torch.int32, device="cpu"
+                )
+            else:
                 last_page_len_host = last_page_len.cpu()
 
             kv_lens_arr_host = get_seq_lens(indptr_host, last_page_len_host, page_size)