add setter for trainer processor (#33911)
* add setter for trainer processor
* Update src/transformers/trainer.py
Co-authored-by: Quentin Gallouédec <45557362+qgallouedec@users.noreply.github.com>
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Co-authored-by: Quentin Gallouédec <45557362+qgallouedec@users.noreply.github.com>
