Microbenchmarking for lxu_cache_populate_byte. (pytorch#936)

Summary:
Pull Request resolved: pytorch#936

While internally there are lxu_cache_find_uncached, lxu_cache_insert_byte kernels (with other kernels that are small portion), this is only exposed interface in fbgemm as of now.

Add benchmark at that level.

Reviewed By: jianyuh

Differential Revision: D34195017

fbshipit-source-id: a38e3d5e9f917ae186faa92b8e240c4624b2bc76