Update qengine flag in python to string (#26620)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26620
This change updates torch.backend.quantized.engine to accept string ("fbgemm"/"qnnpack"/"none" for now).
set_qengine and get_qengine return an int which represents the at::QEngine enum
Test Plan:
python test/test_torch.py
Imported from OSS
Differential Revision: D17533582
fbshipit-source-id: 5103263d0d59ff37d43dec27243cb76ba8ba633f
