fix generate mask fpn, test=develop (#19301)

paddle/fluid/operators/detection/generate_mask_labels_op.cc

@@ -305,10 +305,10 @@ class GenerateMaskLabelsKernel : public framework::OpKernel<T> {
     PADDLE_ENFORCE_EQ(gt_segms->lod()[0].size() - 1, n);
 
     int mask_dim = num_classes * resolution * resolution;
-
-    mask_rois->mutable_data<T>({rois->numel(), kBoxDim}, ctx.GetPlace());
-    roi_has_mask_int32->mutable_data<int>({rois->numel(), 1}, ctx.GetPlace());
-    mask_int32->mutable_data<int>({rois->numel(), mask_dim}, ctx.GetPlace());
+    int roi_num = rois->lod().back()[n];
+    mask_rois->mutable_data<T>({roi_num, kBoxDim}, ctx.GetPlace());
+    roi_has_mask_int32->mutable_data<int>({roi_num, 1}, ctx.GetPlace());
+    mask_int32->mutable_data<int>({roi_num, mask_dim}, ctx.GetPlace());
 
     framework::LoD lod;
     std::vector<size_t> lod0(1, 0);