replaced assert with raise ValueError for t5, switch_transformers, pix2struct, mt5, longt5, gptsan_japanese.
Author: susnato
Pull request: yes

# What does this PR do?

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As said by @sanchit-gandhi, [here](https://github.com/huggingface/transformers/pull/21785#discussion_r1184787328) this PR replaces `assert` with `raise ValueError` for models - t5, switch_transformers, pix2struct, mt5, longt5, gptsan_japanese.

## 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:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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@sanchit-gandhi 

Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hi @sanchit-gandhi just pushed the change that you requested.

Awesome thanks - requesting a final review!

Hi @amyeroberts, just pushed the changes you requested! 
Let me know if any more changes are needed or not.

Merging as errors are unrelated to this PR and have been resolved on main