Update the original mapping in _LazyConfigMapping to fix AutoTokenizer registration
Author: lolipopshock
Pull request: yes

# What does this PR do?

Currently when we want to register a new config+tokenizer+model, per [the instructions](https://huggingface.co/docs/transformers/model_doc/auto), 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).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@n1t0, @LysandreJik

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18491). 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](https://github.com/huggingface/transformers/blob/ab2006e3d6db88654526a4169e65d4bfc52da2e3/tests/models/auto/test_tokenization_auto.py#L234) of this feature we could investigate more.

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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.