Get rid of multiple inheritence in test_torch (#39110)
Summary:
`_TestTorchMixin` is base class which is instantiated across multiple types.
It was inherited from `object` in order to hide it from unittest test discovery mechanism.
But this approach makes it almost impossible to use static code analyzer on the class.
This PR implements alternative approach by hiding base class into inner class, per https://stackoverflow.com/a/25695512
Change imported class access path in `test_cuda.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39110
Test Plan:
run `test_torch.py --discover-tests` and `test_cuda.py --discover-tests` before and after change:
```
$ python test_torch.py --discover-tests|md5sum
2ca437bb5d65700763ce04cdacf6de3e -
$ python test_cuda.py --discover-tests|md5sum
b17df916fb0eeb6f0dd7222d7dae392c -
```
Differential Revision: D21759265
Pulled By: malfet
fbshipit-source-id: b01b06111469e551f7b78387449975e5248f6b9e
