quant: fix bug with copy.deepcopy of FX prepared quantization models (#46895)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46895
Bug: models after the FX graph mode quant prepare step lost information,
such as the extra attributes defined in `Quantizer.save_state`,
if the user performed `copy.deepcopy` on them. The information was lost
because `GraphModule` does not copy attributes which are not present on
`nn.Module` by default.
Fix: define a custom `__deepcopy__` method on observed models and
whitelist the attributes we care about.
This is needed because users sometimes run `copy.deepcopy` on their
models during non-quantization related preparations, and we should make
sure that quantization related state survives these calls.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx.test_deepcopy
python test/test_quantization.py TestQuantizeFx.test_standalone_module
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D24556035
fbshipit-source-id: f7a6b28b6d2225fa6189016f967f175f6733b124
