graph mode: add quantization type enum (#39795)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39795
Replaces the `is_dynamic` bool by enums in Python and c++
graph quantization code. This makes the code more readable
and will make it easier to modify for adding QAT logic in the future.
Test Plan:
CI, as well as
```
python test/test_quantization.py TestQuantizeDynamicScript
python test/test_quantization.py TestQuantizeScriptJitPasses
```
Imported from OSS
Differential Revision: D21981643
fbshipit-source-id: d475760407bcc794aeae92a2c696bac4acda843d
