Avoid printing erroneous warning about "MIOpen not found" for ROCm builds (#33837)
Summary:
Older versions of MIOpen (<=2.2) don't have the `miopenGetVersion` api, but MIOpen is always a part of the ROCm builds, so do NOT set `lib` to None for ROCm builds. `__cudnn_version` will be `None` for older versions of MIOpen.
Setting `lib` to `None` ends up printing the following erroneous warning when running unit tests:
```
/root/.local/lib/python3.6/site-packages/torch/backends/cudnn/__init__.py:120: UserWarning: cuDNN/MIOpen library not found. Check your LD_LIBRARY_PATH
}.get(sys.platform, 'LD_LIBRARY_PATH')))
```
Eg.: https://ci.pytorch.org/jenkins/job/pytorch-builds/job/py3.6-clang7-rocmdeb-ubuntu16.04-test2/18387/consoleFull
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33837
Differential Revision: D20369285
Pulled By: xw285cornell
fbshipit-source-id: e82e6f8f5bccb486213cf868f40aece41ce11f98
