Add default arg to `prepare_qat` mapping. (#28193)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28193
Fixes #28015
Test Plan: Imported from OSS
Differential Revision: D17973121
Pulled By: z-a-f
fbshipit-source-id: 03b3f70c70b89060c1f03d7ed8ab6002fe60bd49
