[Werror] fix Werror-maybe-uninitialized in roi_align_grad_kernel (#51633)

* fix Werror in roi_align_grad_kernel

* adopt a better way

paddle/phi/kernels/cpu/roi_align_grad_kernel.cc

@@ -81,7 +81,7 @@ void RoiAlignGradKernel(const Context& dev_ctx,
                         int sampling_ratio,
                         bool aligned,
                         DenseTensor* dx) {
-  auto in_dims = x.dims();
+  const auto& in_dims = x.dims();
   int channels = in_dims[1];
   int height = in_dims[2];
   int width = in_dims[3];