Open source fakefp16 BatchMatMul op (#36789)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36789
ATT. This should enable test_batchmatmul_nnpi_fp16.py test.
Test Plan:
```
buck test caffe2/caffe2/fb/fbgemm:
```
Reviewed By: hyuen
Differential Revision: D21084876
fbshipit-source-id: d2b4a4c44ad5cf454cefbe6a4cafdf110c6d35cb
