Copy XNNPACK quantizer to ARM backend (#2401)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2401

Copy the XNNPACK quantizer from PyTorch Core to the ARM backend. This is done to provide a copy of the code under the ExecuTorch license.

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Reviewed By: digantdesai

Differential Revision: D54848536

fbshipit-source-id: e1f3fa872b42356bb799c9b9d3a89e27d5059ad0