fix bug when falling back to acc32 when weight is prepacked (#18881)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18881
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18878
When the weight is prepacked and it doesn't contain a prepacked weight for acc32, we shouldn't fallback to acc32.
TODO: add unit tests with better coverage
Reviewed By: feiyu1990
Differential Revision: D14778810
fbshipit-source-id: d49a8c4b7c815ab29b77feb53ee730ad63780488
