uniformize processor Mllama
Author: yonigozlan
Pull request: yes

# What does this PR do?
Adds uniformized processors following https://github.com/huggingface/transformers/issues/31911 for Mllama.

- Very small changes in Mllama processor to be coherent with other vlms processors
- Add ProcessorMixin to Mllama processor tests

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## Before submitting
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## Who can review?

@qubvel @molbap 

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