use warning_once instead of warning in Trainer.tokenizer
Author: jzhang533
Pull request: yes

# What does this PR do?

use warning_once instead of warning in Trainer.tokenizer.

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enhancement to #35452 

Fixes <https://discuss.huggingface.co/t/trainer-warning-with-the-new-version/115446>

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Comments:
Thanks for your PR @jzhang533! Some of the changes were already done by https://github.com/huggingface/transformers/pull/35452 which will be merged soon; can you limit your PR to the other changes?