[Kernel] Fix input for flashinfer prefill wrapper. (#7008)

vllm/attention/backends/flashinfer.py

@@ -133,13 +133,20 @@ def begin_forward(self):
                 return
 
             assert self.prefill_wrapper is not None
+            assert self.query_start_loc is not None
             assert self.paged_kv_indices is not None
             assert self.paged_kv_indptr is not None
             assert self.paged_kv_last_page_len is not None
-            self.paged_kv_indices = self.paged_kv_indices.to(self.device)
-            self.paged_kv_indptr = self.paged_kv_indptr.to(self.device)
+            batch_size = self.query_start_loc.shape[0] - 1
+            assert batch_size >= 0
+            # The prefill stage does not read kv cache.
+            # Both paged_kv_indices and paged_kv_last_page_len are empty.
+            # paged_kv_indptr is a zero tensor with size batch_size + 1.
+            self.paged_kv_indptr = torch.zeros(batch_size + 1,
+                                               device=self.device)
             self.paged_kv_last_page_len = self.paged_kv_last_page_len.to(
                 self.device)
+            self.paged_kv_indices = self.paged_kv_indices.to(self.device)
             self.prefill_wrapper.end_forward()
             self.prefill_wrapper.begin_forward(
                 self.query_start_loc, self.paged_kv_indptr,