[T5] Fix init in TF and Flax for pretraining
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #16749


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

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- rag: @patrickvonplaten, @lhoestq
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Library:

- benchmarks: @patrickvonplaten
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- text generation: @patrickvonplaten
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- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

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