fix submodule traversal in FoldPrepackedWeightIntoModule (#29925)
Summary:
similar to https://github.com/pytorch/pytorch/pull/29914
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29925
Differential Revision: D18548029
Pulled By: jerryzh168
fbshipit-source-id: 7b36133454c5190be19380bf125203807ea0b129
