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* Seed get_train_sampler's generator with arg seed to improve reproducibility

and make the world_size<=1 code path more similar to the others

* move test file into trainer test explicitly

* dumb typo

* make style lint happy

* per discussion, switch to data_seed

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

* Per-folder tests reorganization
Co-authored-by: sgugger <sylvain.gugger@gmail.com>
Co-authored-by: Stas Bekman <stas@stason.org>