Turn on exact_dtype by default on test_optim.py (#34825)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/34825
Test Plan: Imported from OSS
Differential Revision: D20498111
Pulled By: great-way
fbshipit-source-id: e689ca40c496b6b4cccb0df30bdae89b2c024f31
