Build Unit Test of SparseRAdam
Summary: We added caffe2 python wrapper and unit test for the SparseRAdam C++ operator.
Test Plan:
Unit test is constructed following the design pattern of [Wngrad optimizer](https://our.intern.facebook.com/intern/diff/D8655724/). Test passed smoothly.
buck test //caffe2/caffe2/python:optimizer_test -- TestSparseRAdam
Test result:
{F221144048}
Reviewed By: wx1988
Differential Revision: D18330650
fbshipit-source-id: e0f4724c2b616b665e2a0fe2e5c3430696cca7ee
