[pytorch] remove boilerplate setQEngine() from PyTorch mobile predictors (#34556)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34556
According to
https://github.com/pytorch/pytorch/pull/34012#discussion_r388581548,
this `at::globalContext().setQEngine(at::QEngine::QNNPACK);` call isn't
really necessary for mobile.
In Context.cpp it selects the last available QEngine if the engine isn't
set explicitly. For OSS mobile prebuild it should only include QNNPACK
engine so the default behavior should already be desired behavior.
It makes difference only when USE_FBGEMM is set - but it should be off
for both OSS mobile build and internal mobile build.
Test Plan: Imported from OSS
Differential Revision: D20374522
Pulled By: ljk53
fbshipit-source-id: d4e437a03c6d4f939edccb5c84f02609633a0698
