[s2s testing] turn all to unittests, use auto-delete temp dirs
Currently in `examples/seq2seq/test_seq2seq_examples.py` many tmp dirs remain uncleaned by the end of the test:

This PR does:
* Now that we have `parameterized`, convert all tests into unittests
* Now that we have unittests, we can use auto-delete tmp dirs easily using `get_auto_remove_tmp_dir()`
* convert 4 test files
* moved out the broken multi_gpu test - working on it separately here https://github.com/huggingface/transformers/pull/7281 - will re-add when it's complete.

@sshleifer 
@sshleifer, one thing I wasn't sure about - since now the temp dir creation/destruction is automated and it's easy to hardwire those by just passing the explicit path `self.get_auto_remove_tmp_dir("/tmp/favoritedir")` when debugging - I removed the prefices that were used in `tempfile.mkdtemp`. Do you still want to have the option of having a specific prefix? With prefix you still have to hunt for the unique ID which changes on every test rerun. This is a much simpler option, IMHO.

To me it sounds it'd be useful during a debug to have one common path prefix for all tmp dirs of the same test and then add suffices instead for the different specific tmp dir of the test. <|||||>Whatever you think is best! <|||||>I think the prefixes are no longer needed. So it's good to go. 