fix(wandb): pass fake dataset to avoid exception in trainer (see #34455)
Author: cezanne-pascal-aracom
Pull request: yes

# What does this PR do?
This PR passes a fake evaluation dataset for the initialisation of the `fake_trainer` of WandB.
This is necessary, since the init of the `Trainer` checks if an `eval_strategy` is set and raises an error if the `eval_dataset` is missing.
This issue is documented in #34701.
The fix here is similar to the one in #34455.


Fixes # (issue)
- Pass `["fake"]` as `eval_dataset` during initialisation of `fake_trainer` for saving purposes of WandB.

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Comments:
Hello @CezaPasc, I believe this should have been fixed with https://github.com/huggingface/transformers/pull/34455

Ah, my bad, I see where this one is different. There are a few files which are changed which shouldn't be; do you mind keeping the diff only to the fix here? Thank you!

@LysandreJik 
It seems like I got the other diffs from rebasing with the upstream.
I reverted them so that the diff should now only be about the changes in this PR again.