Fix CPU implementation of roi_align_op backward (PaddlePaddle#18728)

paddle/fluid/operators/roi_align_op.h

@@ -154,6 +154,8 @@ class CPUROIAlignOpKernel : public framework::OpKernel<T> {
     int width = in_dims[3];
     int rois_num = rois->dims()[0];
 
+    if (rois_num == 0) return;
+
     auto in_stride = framework::stride(in_dims);
     auto roi_stride = framework::stride(rois->dims());
     auto out_stride = framework::stride(out->dims());
@@ -278,6 +280,10 @@ class CPUROIAlignGradOpKernel : public framework::OpKernel<T> {
     const T* out_grad_data = out_grad->data<T>();
     T* in_grad_data = in_grad->mutable_data<T>(ctx.GetPlace());
 
+    auto& dev_ctx = ctx.template device_context<DeviceContext>();
+    math::SetConstant<DeviceContext, T> set_zero;
+    set_zero(dev_ctx, in_grad, static_cast<T>(0));
+
     auto in_stride = framework::stride(in->dims());
     auto roi_stride = framework::stride(rois->dims());
     auto out_stride = framework::stride(out_grad->dims());