[qat]Removed outdated context manager in unit test. (#66274)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66274
Removed outdated context manager in unit test.
* The linked issue (https://github.com/pytorch/pytorch/issues/23825) seemed have been be fixed in 2020.
Test Plan: buck run mode/dev-nosan //caffe2/test:quantization -- -r quantization.eager.test_quantize_eager_qat
Reviewed By: vkuzo
Differential Revision: D29507087
fbshipit-source-id: e8fa04c9527023a5adaf1a012b2c393ce0c5cd97
