[ROCm][V1] Update reshape_and_cache to properly work with CUDA graph padding (#13922)

csrc/cache_kernels.cu

@@ -375,7 +375,7 @@ void reshape_and_cache(
     torch::Tensor& slot_mapping,  // [num_tokens]
     const std::string& kv_cache_dtype, torch::Tensor& k_scale,
     torch::Tensor& v_scale) {
-  int num_tokens = key.size(0);
+  int num_tokens = slot_mapping.size(0);
   int num_heads = key.size(1);
   int head_size = key.size(2);
   int block_size = key_cache.size(3);