Unify Quantization APIs for add, pool and relu (#26586)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26586
Use the backend engine flag to call QNNPACK for quantized ops.
Test Plan: python test/test_quantized.py TestQNNPACKOps
Differential Revision: D17515129
Pulled By: supriyar
fbshipit-source-id: 951e90205aa19581ea006a91d9514fc7a94409ef
