delete critical section in TH*Tensor_addmm (#16889)
Summary:
This was serializing all calls to `addmm` (and any op that used it, in my case `bmm`) in the entire process, and led to downright atrocious performance in the TorchScript threaded runtime. Removing this gives a 2x throughput boost for high-load machine translation inference.
The original justification for this is dubious: there are other `gemm` callsites in the codebase that are not protected by critical sections. And in caffe2 land we never had any issues with nonreentrant BLAS libraries
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16889
Differential Revision: D14008928
Pulled By: jamesr66a
fbshipit-source-id: 498e2133bd6564dba539a2d9751f4e61afbce608
