add rowwise adagrad lp test (#15082)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15082
We didn't have unit test for low-precision rowwise adagrad
Reviewed By: chocjy
Differential Revision: D13300732
fbshipit-source-id: 46e7bdfc82c5a6855eeb6f653c0a96b0b3a20546
