Reorganize examples
# What does this PR do?

This PR reorganizes the examples folder by splitting it in two:
- `examples` that stay in this folder are the base example scripts maintained with the state of the library, expected to work on master. We accept PRs on them and will try our best to fix issues.
- `research-projects` are (often) more complex examples that we don't really maintain. They work on a specific version of the library  (sometimes even a specific commit). We don't accept PRs on them except minor typo fixes *or* PRs from the original authors that want to bring an update to those scripts. Issues opened for those are probably less efficient than directly contacting the authors.

Each example/research project lives in a folder of its own, with its particular requirements in a `requirements.txt` file (instead of a global requirements file as before).

The seq2seq subfolder is less organized than the others, so I did my best to split its research project part from its example part. I made sure all tests are passing and duplicated the needed files, but @stas00 and @patil-suraj please tell me if you something obvious that I missed. We will leave the research-project part as is and clean a bit more the part in the examples in other PRs.


another bit - `examples/seq2seq/test_data` is also used by `research_projects/seq2seq` - perhaps symlink? <|||||>Maybe a hard copy in that case, just in case the data changes/moves on the examples side.<|||||>> Maybe a hard copy in that case, just in case the data changes/moves on the examples side.

Then need to check which specific sub-dirs are needed - if I'm not mistaken it's only `test_data/wmt_en_ro/`

I'd still use a symlink to avoid git repo bloat and this can always be easily fixed if there is a divergence down the road.<|||||>Does this mean we no longer explicitly support pytorch_lightning? <|||||>> Does this mean we no longer explicitly support pytorch_lightning?

I would rather like it if we ditch the custom transformer trainer and just use lightning.