[quant] AO migration of the `fuse_modules.py` (phase 1) (#64913)

[quant] AO migration of the `fuse_modules.py` (phase 1) (#64913)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64913

AO Team is migrating the existing torch.quantization into torch.ao.quantization. We are doing it one file at a time to make sure that the internal callsites are updated properly.
This migrates the fuse_module.py from torch.quantization to torch.ao.quantization.
At this point both locations will be supported. Eventually the torch.quantization will be deprecated.

Test Plan: `buck test mode/dev //caffe2/test:quantization`

Reviewed By: vkuzo

Differential Revision: D30882819

fbshipit-source-id: 1926ad6aa49136aceb5b625dcef4bfde3a2860d4