[Doctest] Fix Perceiver doctest

[younesbelkada]

What does this PR do?

This PR fixes a failing doctest for PerceiverModel. Link to failing job: https://github.com/huggingface/transformers/actions/runs/4002271138/jobs/6869333719

With #21225 being merged, the snippet here:

...
model = PerceiverModel(config, input_preprocessor=preprocessor, decoder=decoder)
# you can then do a forward pass as follows:
tokenizer = PerceiverTokenizer()

has been modified by:

...
model = PerceiverModel(config, input_preprocessor=preprocessor, decoder=decoder)
# you can then do a forward pass as follows:
tokenizer = AutoTokenizer()

As there is no canonical way to automatically load a tokenizer from something which is different than a path, or model id, one should load any default tokenizer by instantiating it using the child class and not using AutoTokenizer.

This PR reverts this change and fixes the doctest

cc @ydshieh 💯

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ydshieh]

Look more than reasonable to me! Thanks. Let Sylvain to see if he has any other thought though, as he is the master of auto!

[sgugger]

Thanks for the fix!