Add num_hidden_layers to t5gemma's top level config
Author: winnie0617
Pull request: yes

# What does this PR do?

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This PR fixes a bug that for generating with cache using T5Gemma. Due to the nested structure of its config class, the field `num_hidden_layers`, which is used to initialize a dynamic cache, only exists under `config.decoder`. 

This PR added a one line change to initialize `config.num_hidden_layers` using `config.decoder.num_hidden_layers`.

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Fixes #41073 


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## Who can review?

@gante @Rocketknight1

Following up on this issue since I encountered the same error. Happy to provide more information or discuss alternative solutions to help get this merged :) 

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: t5gemma

cc @gante ?

(but maybe we should handle this in the cache class instead of adding a new attribute only for this purpose ?)

[This PR](https://github.com/huggingface/transformers/pull/40939) is the actual fix 🤗 

(this has been in review for a while, I'll ping a reviewer)