What does this PR do?

Currently when we want to register a new config+tokenizer+model, per the instructions, it seems we should do the following:

from transformers import AutoConfig, AutoModel

AutoConfig.register("new-model", NewModelConfig)
AutoTokenizer.register(NewModelConfig, TokenizerSlow, TokenizerFast)
AutoModel.register(NewModelConfig, NewModel)

AutoTokenizer.from_pretrained("xxx") # <--- error  `Unrecognized configuration class <xxx> to build an AutoTokenizer.`

However, there is one potential bug in the current AutoTokenizer registration code:

• In https://github.com/huggingface/transformers/blob/280db2e39c1e586389df4e46f2b895fc092911bb/src/transformers/models/auto/tokenization_auto.py#L605, AutoTokenizer will config_class_to_model_type to determine whether the corresponding config is registered in the input config.
• The config_class_to_model_type function checks the CONFIG_MAPPING_NAMES to find the newly register config class. https://github.com/huggingface/transformers/blob/280db2e39c1e586389df4e46f2b895fc092911bb/src/transformers/models/auto/configuration_auto.py#L438
• However, according to https://github.com/huggingface/transformers/blob/280db2e39c1e586389df4e46f2b895fc092911bb/src/transformers/models/auto/configuration_auto.py#L781 , after registering a config, the CONFIG_MAPPING only updates the _extra_content but not the original mapping or CONFIG_MAPPING_NAMES in this case https://github.com/huggingface/transformers/blob/280db2e39c1e586389df4e46f2b895fc092911bb/src/transformers/models/auto/configuration_auto.py#L492 . That is to say, the config_class_to_model_type cannot find the newly registered config in this case, and will throw an error Unrecognized configuration class <xxx> to build an AutoTokenizer.

A temporary local hot fix can be:

from transformers import AutoConfig, AutoModel
from transformers.models.auto.configuration_auto import CONFIG_MAPPING_NAMES

AutoConfig.register("new-model", NewModelConfig)
CONFIG_MAPPING_NAMES["new-model"] = NewModelConfig.__name__
AutoTokenizer.register(NewModelConfig, TokenizerSlow, TokenizerFast)
AutoModel.register(NewModelConfig, NewModel)

But thought it would be better to fix it upstream.

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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• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

@n1t0, @LysandreJik

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

cc @sgugger

Do you have a full example of the error you are reporting I could run? I am unable to reproduce it. Something like the test of this feature we could investigate more.

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