[Pytorch] Remove assumption forward exists in freeze_module (#52918)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52918
Freeze_module seems to operate under the assumption that forward always exists. This isnt true, so the change first checks for existence then retrieves the function.
ghstack-source-id: 123215242
Test Plan: Try freezing something with and without forward.
Reviewed By: dhruvbird
Differential Revision: D26671815
fbshipit-source-id: d4140dad3c59d3d20012143175f9b9268bf23050
