fix roi_align_op cpu backward's bug (#18825)

[cherry pick]fix roi_align_op cpu backward's bug

Author: FDInSky

paddle/fluid/operators/roi_align_op.h

@@ -256,13 +256,15 @@ class CPUROIAlignGradOpKernel : public framework::OpKernel<T> {
     auto spatial_scale = ctx.Attr<float>("spatial_scale");
     auto sampling_ratio = ctx.Attr<int>("sampling_ratio");
     auto in_dims = in->dims();
-    if (!in_grad) {
-      return;
-    }
+
     int channels = in_dims[1];
     int height = in_dims[2];
     int width = in_dims[3];
     int rois_num = rois->dims()[0];
+
+    if (!in_grad) {
+      return;
+    }
     Tensor roi_batch_id_list;
     roi_batch_id_list.Resize({rois_num});
     int* roi_batch_id_data =
@@ -276,14 +278,21 @@ class CPUROIAlignGradOpKernel : public framework::OpKernel<T> {
       }
     }
 
-    const T* rois_data = rois->data<T>();
-    const T* out_grad_data = out_grad->data<T>();
-    T* in_grad_data = in_grad->mutable_data<T>(ctx.GetPlace());
-
+    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));
 
+    int output_grad_size = out_grad->numel();
+
+    if ((!out_grad->IsInitialized()) || (output_grad_size <= 0)) {
+      return;
+    }
+
+    const T* rois_data = rois->data<T>();
+    const T* out_grad_data = out_grad->data<T>();
+    T* in_grad_data = in_grad->mutable_data<T>(ctx.GetPlace());
+
     auto in_stride = framework::stride(in->dims());
     auto roi_stride = framework::stride(rois->dims());
     auto out_stride = framework::stride(out_grad->dims());