Disable flaky tests test_DistributedDataParallel and test_backend_group for ROCm (#33211)
Summary:
Getting intermittent error in CI runs:
**TestDistBackend.test_DistributedDataParallel**
```
02:36:32 File "/var/lib/jenkins/.local/lib/python3.6/site-packages/torch/serialization.py", line 442, in _legacy_save
02:36:32 pickler.dump(obj)
02:36:32 AttributeError: Can't pickle local object 'Module._replicate_for_data_parallel.<locals>.zero_grad'
```
Some CI runs where it failed:
https://ci.pytorch.org/jenkins/job/pytorch-builds/job/py3.6-clang7-rocmdeb-ubuntu16.04-test2/16163/console
https://ci.pytorch.org/jenkins/job/pytorch-builds/job/py3.6-clang7-rocmdeb-ubuntu16.04-test2/16165/console
**TestDistBackend.test_backend_group**
```
test_backend_group (__main__.TestDistBackend) ... Memory access fault by GPU node-5 (Agent handle: 0x265c670) on address 0x7fded754a000. Reason: Page not present or supervisor privilege.
```
Some CI runs where it failed:
https://ci.pytorch.org/jenkins/job/pytorch-builds/job/py3.6-clang7-rocmdeb-ubuntu16.04-test2/16288/console
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33211
Differential Revision: D19849089
Pulled By: bddppq
fbshipit-source-id: 5e997653cc344f4c6819d46bedc6d3bd75b5d854
