make SpeechT5 doc examples deterministic
Author: hollance
Pull request: yes

# What does this PR do?

Fixes an issue with the doc examples for SpeechT5. Due to the dropout layer being used in inference mode, the predicted sequence length is not always the same, which causes the doc tests to fail. Setting the seed fixes this.

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Fixes # (issue)


## Before submitting
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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts and @NielsRogge
- 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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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Thank you @hollance . Before I could merge, could you change the docstrings that contain(s)

```
dataset = load_dataset(...) 
```
to 
```
dataset = load_dataset(...)   # doctest: +IGNORE_RESULT
```

🙏 Thank you.

@ydshieh If I do this, `make fixup` will wrap this code like so:

```python
        >>> dataset = load_dataset(
        ...     "hf-internal-testing/librispeech_asr_demo", "clean", split="validation"
        ... )  # doctest: +IGNORE_RESULT
```

Is that OK?

> @ydshieh If I do this, `make fixup` will wrap this code like so:
> 
> ```python
>         >>> dataset = load_dataset(
>         ...     "hf-internal-testing/librispeech_asr_demo", "clean", split="validation"
>         ... )  # doctest: +IGNORE_RESULT
> ```
> 
> Is that OK?

totally OK :-)