Move specific quantizer related things outside of main quant code base

Summary:
X-link: https://github.com/pytorch/pytorch/pull/107259

Currently in quantizer/quantize_pt2e we import things from specific quantizers (XNNPACKQuantizer, QuantizationConfig) etc.
this PR removes them so it's clearer that they are not part of the core quantization code base

This PR also removed get_supported_operators from main Quantizer since we haven't seen a clear need for this API

Reviewed By: andrewor14

Differential Revision: D48340367

fbshipit-source-id: 368cca59c13b36049f33a8b1ca7842476daf72c5