[Quant][fx] Implement BackendConfig (part 1) (#81469)
Summary: Following https://github.com/pytorch/pytorch/pull/78452
and https://github.com/pytorch/pytorch/pull/79066, this commit
is part 1 of the broader effort to replace `backend_config_dict`
with a python config object, a more formal and robust API that
leads to better user experience. Note that there is no change in
behavior in this commit by itself. A future commit (part 2) will
replace all existing usages of `backend_config_dict` with the
`BackendConfig` object added in this commit.
Test Plan:
python test/test_quantization.py TestBackendConfig
Reviewers: jerryzh168
Subscribers: jerryzh168
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81469
Approved by: https://github.com/jerryzh168
