[quant][bug] Fixing the mapping getter to return a copy (#50297)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50297
Current implementation has a potential bug: if a user modifies the quantization mappings returned by the getters, the changes will propagate.
For example, the bug will manifest itself if the user does the following:
```
my_mapping = get_default_static_quant_module_mappings()
my_mapping[nn.Linear] = UserLinearImplementation
model_A = convert(model_A, mapping=my_mapping)
default_mapping = get_default_static_quant_module_mappings()
model_B = convert(model_B, mapping=default_mapping)
```
In that case the `model_B` will be quantized with with the modified mapping.
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D25855753
Pulled By: z-a-f
fbshipit-source-id: 0149a0c07a965024ba7d1084e89157a9c8fa1192
