gemm_tr, gemm_ad do not use matrixmultiply

It seems that `gemm_tr` and `gemm_ad` currently do not leverage `matrixmultiply` for larger matrices. I noticed when I was profiling after making some changes to some of my performance-sensitive code. In fact, pre-computing `let a_t = a.transpose()` and calling `gemm(1.0, &a_t, &b, 1.0)` was significantly faster.