fix bug when falling back to acc32 when weight is prepacked (#18974)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18974
When the weight is prepacked and it doesn't contain a prepacked weight for acc32, we shouldn't fallback to acc32.
Reviewed By: bddppq
Differential Revision: D14814067
fbshipit-source-id: aec917322de695e283f0aca1e930c5603d196404
