[accuracy minifier] cast copied model rather than update the original model (#101901)
This is the fix Ed found during the break of the summit :)
I think I'd better to split it out of https://github.com/pytorch/pytorch/pull/99773 so people don't need to patch that PR to run the repro.py
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101901
Approved by: https://github.com/ezyang