Fix select input error when handle 0 dim Tensor (#44251)

paddle/fluid/operators/assign_op.h

@@ -71,7 +71,7 @@ class AssignFunctor {
  private:
   void copy_tensor(const framework::LoDTensor &lod_tensor,
                    framework::LoDTensor *out) const {
-    if (lod_tensor.numel() == 0) return;
+    if (!lod_tensor.IsInitialized()) return;
     auto &out_tensor = *out;
     paddle::framework::TensorCopy(lod_tensor, lod_tensor.place(), &out_tensor);
     out_tensor.set_lod(lod_tensor.lod());