Guide to create custom models in Spanish
# What does this PR do?

<!--

This PR translates the guide on h[ow to create custom models](https://github.com/huggingface/transformers/blob/main/docs/source/en/create_a_model.mdx) to the Spanish language, as talked in #15947 .
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Linked to #15947


## Before submitting
- [X ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

@omarespejel said he will review, otherwise anyone 😁.

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the suggestions, I've committed all the changes you proposed.<|||||>@ignacioct muchas gracias por tu PR! 🤗 Vamos a hacerle merge. If there is any other doc that you would like to translate you can tell me. 

@sgugger LGTM 🔥