[aten] remove stmt unreachable, variable never used warnings (#34017)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34017
Remove warning
```
caffe2/aten/src/THC/generic/THCTensorMathBlas.cu(437): warning: statement is unreachable
caffe2/aten/src/THC/generic/THCTensorMathBlas.cu(271): warning: variable "transpose_m1" was set but never used
caffe2/aten/src/THC/generic/THCTensorMathBlas.cu(271): warning: variable "transpose_m2" was set but never used
```
Test Plan: CI
Reviewed By: ngimel
Differential Revision: D20181179
fbshipit-source-id: 3665912ba55bffbd8b4555f8a6803e57a502c103
