Should throw an error if `repo_id` is not valid

Related to https://github.com/huggingface/transformers/issues/18691.

Currently if repo_id value is set to a wrong value, there is no validation before calling cache/the hub. Typically, if the value is a path "./resources/ltp" (by mistake from the user) in hf_hub_download, then it will raise a FileNotFoundError:

FileNotFoundError: [Errno 2] No such file or directory: '/home/chaizhihua/.cache/huggingface/hub/models--.--resources--ltp/refs/main'

It would be good to have a helper to validate a repo_id and raise a custom exception if it’s obvious than it is not a valid repo_id. As a first step I would consider any value in the form “xxx/xxx” or “xxx”.

[LysandreJik]

I’d also definitely avoid datasets/user/repo_name. I don’t think it’ll break anything in the wild that we mentioned was supported. This should really be handled by repo_type="dataset".

Sounds good for validation!

[LysandreJik]

(re last comment: it would have broken transformers, shame on me)