Add control for observers in Fake-quantize module (#27113)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27113
Fix bug in fake quant control of observer and fake-quantize operations.
Add test to ensure that features work as expected
ghstack-source-id: 91071181
Test Plan: buck test mode/dev-nosan caffe2/test:fake_quant -- test_fake_quant_control
Differential Revision: D17678875
fbshipit-source-id: 2912ad8b6e674daa1d129f7a7c6f27d8c1b4f93b
