Add ability to run Gemma 2 models without post layer norm

[amer-sinha]

What does this PR do?

adds the ability for Gemma2 models to run without post attention layer normalization and post feedforward layer normalization.

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gemma2, t5gemma

[vasqu]

Are there any models that use this? We usually only support features that are already included in (to be) published models

[Cyrilvallez]

Indeed! As @vasqu said, the best is to create a new model with modular (it will require very little code and efforts with modular)! 🤗