Make module swap the main QAT flow again

[andrewor14]

Summary: Following https://github.com/pytorch/ao/issues/987, this commit makes module swap the main QAT flow today. We remove all tensor subclass fake quantize injection logic since this is not needed in both the long term and the short term plans for QAT. In the short term, we will continue to use a full module swap flow, and only migrate to the long term flow once there is general distributed support for tensor subclasses and when tensor subclass composability provides meaningful benefits.

Test Plan: python test/quantization/test_qat.py

[pytorch-bot[bot]]

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As of commit 208fd4ed4b40a142928ccb5c642b69f6a6868a84 with merge base 5a4857e3d7d99808ad9b9a8f067ff5e53da4daca (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[andrewor14]

Closing this in favor of https://github.com/pytorch/ao/pull/1019 (exact same PR)