Fix model parallelism for ByT5
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Fixes #20879

Only assign device_map explicitly in parallelize() to model encoder and decoder if it was explicitly passed in from the caller. The encoder and decoder will automatically create a device_map if None is passed in, so the original code was redundant.

ByT5 has a much bigger encoder than decoder, so assuming that the two are the same size (and can use the same device_map) is not correct and results in an assertion error.

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## Who can review?

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts and @NielsRogge
- speech models: @sanchit-gandhi

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger and @stevhliu

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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@ArthurZucker, @younesbelkada
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20880). All of your documentation changes will be reflected on that endpoint.<|||||>Hey, parallelise is very deprecated, but I believe other models might also benefit from this if it is a fix no?
<|||||>Wasn't aware, is the recommendation to use accelerate now?

This fix only affects models with more encoder than decoder blocks. I'm pretty sure this is rare (only seen it done with ByT5, which I am using for character sensitive tasks)<|||||>Yes, the recommendation is to use Accelerate for this form of parallelism (which Accelerate supports for all T5 models), the old API is on its way to be deprecated and won't be maintained.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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