add support for real4bits quant (#25426)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25426
Add embedding table 4bit quantization support.
* add the conversion from fp32 to int4.
* using brew to pass the context so that the 4bit operators are added when generating the predictor net.
Reviewed By: kennyhorror, chocjy
Differential Revision: D16859892
fbshipit-source-id: a06c3f0b56a7eabf9ca4a2b2cb6c63735030d70b
