Rename fbgemm quantized operators to generic `quantized` ops (#25338)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25338
As an effort to unify fbgemm and qnnpack at the dispatcher level, we need to have a generic name for the quantized backed ops.
Currently FBGEMM is guarded by the USE_FBGEMM macro and QNNPACK uses USE_QNNPACK.
TBD: Use compile time macro or run_time to switch between fbgemm and qnnpack.
ghstack-source-id: 89454244
Test Plan: buck test caffe2/test:quantized
Differential Revision: D17097735
fbshipit-source-id: 447112a7a421387724d3e29b8fd8412dfb1c373a
