[examples tests on multigpu] resolving require_torch_non_multi_gpu_but_fix_me
A while ago I added `@require_torch_non_multi_gpu_but_fix_me` to quickly allow us to start running example tests on multigpu, so this PR resolves that temporary band-aid.

This PR:
* fixes a few tests to make them run on multi-gpu
* removes the decorator where it's not needed after testing that it works
* leaves the dropped legacy tests untouched - since they don't run on CI
* eliminates  `@require_torch_non_multi_gpu_but_fix_me` from existence since it's no longer needed
* the only test I couldn't figure out is https://github.com/huggingface/transformers/issues/10560 but it's not worse off than it was - added some refactoring to it and prepared it for multi-gpu if someone knows how to fix it

Note: 2 slow tests in `examples/test_examples.py` currently fail because of yet another missed thing in ported `run_seq2seq.py` - but these should be resolved by https://github.com/huggingface/transformers/pull/10551 once that one is merged, so we can merge this PR after it.

@LysandreJik 
