Add layout options to gemm
Summary:
We were only benchmarking `row-major x row-major` gemms (also called
`TT` or `transpose-transpose`, because FORTRAN), which is actually not the
common case; `nn.Linear` will use column-major layouts for weights, which means
`TN` is actually much more common.
Reviewed By: adamomainz
Differential Revision: D63714661
fbshipit-source-id: 735c25c59ddeb6596afd9b19f463af92036a830b
