graph mode qat: make fake_quantize scriptable (#39750)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39750
Add a test to make the default QAT qconfig scriptable, and fix
all the errors.
Test Plan:
```
python test/test_quantization.py TestQATScript.fake_quant_scriptable
```
Imported from OSS
Differential Revision: D21975879
fbshipit-source-id: 8c48ad9f24b2c941d2267cb53eb70ebecd103744
