Better support for collection retrieval

[gsaltintas]

Hello,

Thank you for all the updates to the api, it has been very useful. When creating models or datasets I often pass a relevant collection too. Currently all functions associated with the huggingface_hub.HfApi() expect collection_slug, which is a unique id exposed by the api and not accessible in the web interface. Unfortunately, there is no good way of retrieving the slug.

I think the cleanest would be to make the collection title unique and pass that to these functions similar to how we retrieve models or datasets, i.e. namespace/collection-name or at least if api. get_collection could support this format one can first check if the collection exists to retrieve its slug.

This is the current workaround I am currently using:

import huggingface_hub as hf_hub

def create_or_get_collection_slug(hf_collection: str) -> str:
    """Return the slug for a collection identified by 'namespace/title', creating it if needed.    """
    api = hf_hub.HfApi()
    namespace, title = hf_collection.split("/", 1) if "/" in hf_collection else (None, hf_collection)

    existing = next(
        (col for col in api.list_collections(owner=namespace) if col.title == hf_collection),
        None,
    )
    if existing:
        return existing.slug

    collection = api.create_collection(title=title, namespace=namespace, exists_ok=True)
    return collection.slug

Is your feature request related to a problem? Please describe.
This is related to the general improvements listed under #3702