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[WIP][examples/seq2seq] move old s2s scripts to legacy (#10136)

* move old s2s scripts to legacy

* add the tests back

* proper rename

* restore

* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Stas Bekman <stas@stason.org>
Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>