move half<->float conversions to oss operators (#17548)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17548
expose half float operators to OSS
common/math/Float16.h is the original implementation
this is substituted by caffe2/c10/util/Half.h
from the comments seems like the both implementations don't handle denormals
Reviewed By: jspark1105
Differential Revision: D14244200
fbshipit-source-id: f90ba28c5bf6a2b451b429cc4925b8cc376ac651
