[Quant][fx] Add get_default_qconfig_mapping
Summary: This follows https://github.com/pytorch/pytorch/pull/78452,
which replaced the qconfig_dict with QConfigMapping. This PR
additionally replaces get_default_*qconfig_dict with
get_default_*qconfig_mapping. For backward compatibility, we
deprecate the old functions instead of removing them.
Test Plan:
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo, supriyar
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79618
Approved by: https://github.com/jerryzh168
