fix examples/rag imports, tests
Before
```bash
pytest examples/rag
```
fails with
```
================================================================ ERRORS =================================================================
______________________________________ ERROR collecting examples/rag/test_distributed_retriever.py ______________________________________
ImportError while importing test module '/Users/shleifer/transformers_fork/examples/rag/test_distributed_retriever.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
examples/rag/test_distributed_retriever.py:26: in <module>
    from examples.rag.distributed_retriever import RagPyTorchDistributedRetriever  # noqa: E402 # isort:skip
E   ModuleNotFoundError: No module named 'examples.rag'
```

After, the same command passes.

The fix was to change
`from examples.rag.file_name -> from filename` 
and add the same `sys.path` magic that `examples/seq2seq/` uses.

### TODO
- test the scripts in `examples/rag/README.md` after this change

@stas00 does this look reasonable? Am I missing anything you did for `examples/seq2seq`?<|||||>yes, except you shouldn't need 
```
sys.path.append(os.path.join(os.getcwd()))  # noqa: E402 # noqa: E402 # isort:skip
```
that's what `__init__.py` already did for you.

and once removed, all those subsequent import `# noqa:` comments can be removed too. (except PL import)

besides using `cwd` is a bad idea - who knows where the script is invoked from and not just from the same dir as the script itself. Use `__file__` instead, which is deterministic. but it's not needed here.
<|||||>Scripts work, will merge on CI success.<|||||>btw, you can also remove most of the `# noqa: E402 # isort:skipq` as they are no longer needed.