[quant] Update get_default_qat_qconfig to return the fused observer+fake_quant module (#62702)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62702
Expose the qconfig to the user to speed up training by leveraging the fused module.
The module currently supports per-tensor/per-channel moving avg observer and fake-quantize.
For details on perf benefits, refer to https://github.com/pytorch/pytorch/pull/61691
Test Plan: Imported from OSS
Reviewed By: raghuramank100
Differential Revision: D30093719
fbshipit-source-id: b78deb7810f5b597474b9b9a0395d361d04eb46a
