Revert "fix(mgb/opr): change EQ opr's backward_graph to nullptr instead of InvalidGrad"

This reverts commit 635533764101de2336ee0216bc51018aef59578d.

GitOrigin-RevId: 6897f6d45d8fda1ee671f5d24ec314b8bfb76a48

src/opr/impl/dnn/roi_align.cpp

@@ -42,9 +42,6 @@ SymbolVar ROIAlignForward::make(SymbolVar src, SymbolVar rois,
 
 #ifdef MGB_ENABLE_GRAD
 MGB_IMPL_OPR_GRAD(ROIAlignForward) {
-    if (out_grad[1]) {
-        return InvalidGrad::make(opr, wrt_idx);
-    }
     if (wrt_idx == 0) {
         // wrt src
         SymbolVar grad =

src/opr/impl/dnn/roi_pooling.cpp

@@ -86,7 +86,7 @@ size_t ROIPoolingForward::get_workspace_size_bytes(
 
 #ifdef MGB_ENABLE_GRAD
 MGB_IMPL_OPR_GRAD(ROIPoolingForward) {
-    if (out_grad[1] || wrt_idx == 2) {
+    if (wrt_idx == 2) {
         return InvalidGrad::make(opr, wrt_idx);
     }
     if (wrt_idx == 0) {