Use _BaseAutoModelClass's register method
Author: fadynakhla
Pull request: yes

# What does this PR do?
Switching `_BaseAutoModelClass`'s `from_pretrained` and `from_config` to use the register classmethod that it defines rather than using the `_LazyAutoMapping` register method directly. This makes use of the additional consistency check within `_BaseAutoModelClass`'s register method.


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## Before submitting
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section? Yes
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Discussed briefly in [#24737 ](https://github.com/huggingface/transformers/issues/24737)
- [ ] Did you make sure to update the documentation with your changes? No public methods/classes changed
- [ ] Did you write any new necessary tests? None necessary


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. @sgugger

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Comments:
> Thanks a lot!

Glad to help!

Also, what are your thoughts on adding some typehinting to the `_model_mapping` variable e.g. 
```
class _BaseAutoModelClass:
    # Base class for auto models.
    _model_mapping: Optional["_LazyAutoMapping"] = None
```
instead of
```
class _BaseAutoModelClass:
    # Base class for auto models.
    _model_mapping = None
```
it took quite some time during our discussion yesterday for me to track down what type of object `_model_mapping` was

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

This is all internal code, so we don't really document types as rigorously as in public facing classes :-)

Sounds good just thought I'd ask