[BugFix] Fix zero workspace returned by CUB size query under CUDA Graph in MoE dispatch (#5087)

* fix bug about CubKeyValueSorter::run

* pre-commit and add comment

* pre-commit

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix precommit

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: YuBaoku <49938469+EmmonsCurse@users.noreply.github.com>

custom_ops/gpu_ops/moe/fused_moe_imp_op.h

@@ -16,8 +16,8 @@
  */
 
 #pragma once
-#include <string>
 #include <sstream>
+#include <string>
 #include "cub/cub.cuh"
 
 namespace phi {
@@ -45,7 +45,10 @@ class CubKeyValueSorter {
   size_t getWorkspaceSize(const size_t num_key_value_pairs,
                           bool descending = false) {
     num_key_value_pairs_ = num_key_value_pairs;
-    size_t required_storage = 0;
+    // Initialize to 1 as workaround: under CUDA Graph capture, CUB may not
+    // write to required_storage, and 1 is the minimum expected size in that
+    // scenario.
+    size_t required_storage = 1;
     int* null_int = nullptr;
     if (descending) {
       cub::DeviceRadixSort::SortPairsDescending(NULL,

custom_ops/gpu_ops/moe/moe_dispatch.cu

@@ -87,6 +87,13 @@ void MoeDispatchKernel(
   int8_t *sorter_ws_ptr = reinterpret_cast<int8_t *>(ws_ptr + bytes);
   int *permuted_experts_ =
       reinterpret_cast<int *>(sorter_ws_ptr + sorter_ws_size_bytes);
+  // If expected_ws_size > workspace_size ever occurs in sorter_.run (which
+  // should be practically impossible), there is a contiguous, currently unused
+  // region (permuted_experts_) right after sorter_ws_ptr. In practice, this
+  // region is larger than what cub::DeviceRadixSort::SortPairs requires.
+  // However, relying on this to “work” after canceling the assertion is unsafe:
+  // it constitutes undefined behavior, and there is no guarantee it will remain
+  // correct across inputs, CUDA/CUB versions, or architectures.
   int *permuted_rows_ = permuted_experts_ + num_moe_inputs;
 
   int *topk_idx_ptr = topk_idx->data<int>();