Fix type checking errors in torch/quantization/fx/qconfig_utils.py (#66428)
Summary:
- [x] Fix the Pyre type checking errors in `torch/quantization/fx/qconfig_utils.py`
```
torch/quantization/fx/qconfig_utils.py:241:46 Incompatible variable type [9]: prepare_custom_config_dict is declared to have type `Dict[str, typing.Any]` but is used as type `None`.
torch/quantization/fx/qconfig_utils.py:267:46 Incompatible variable type [9]: convert_custom_config_dict is declared to have type `Dict[str, typing.Any]` but is used as type `None`.
torch/quantization/fx/qconfig_utils.py:284:43 Incompatible variable type [9]: fuse_custom_config_dict is declared to have type `Dict[str, typing.Any]` but is used as type `None`.
```
Fixes the issue: [MLH-Fellowship/pyre-check/issues/73](https://github.com/MLH-Fellowship/pyre-check/issues/73)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66428
Reviewed By: grievejia
Differential Revision: D31545215
Pulled By: 0xedward
fbshipit-source-id: 767ae7888854c2eec2ecf14855a5b011110b9271
