Make get_model_builder public

[datumbox]

This PR makes it possible to get a model builder method by its name.

This is useful because a model might be registered under a different name than the method’s name (see quantization models, for example quantized_resnet50) and thus fetching it by indexing the module __dict__ might not be possible. This PR addresses feedback received internally by MobileVision and will simplify the integration with fastai (see fastai/fastai#3789).

[NicolasHug]

Changing find_model into get_model_builder was originally suggested in #6333 (comment) and in #6333 (comment) but it wasn't accepted at the time. What is the reason to change it now?

Also considering model buillders are now available, does it still make sense to expose get_model()? IIUC get_model(name, args) is now just get_model_builder(name)(args), so we would have 2 ways of doing the same thing?

[datumbox]

@NicolasHug Just saw your message.

What is the reason to change it now?

It was a private method named similarly to [find()](https://github.com/pytorch/vision/blob/main/torchvision/prototype/datasets/_api.py#L38) from datasets. Now that we publicly expose it, we can adopt your original proposal.

so we would have 2 ways of doing the same thing?

I don't mind particularly offering both and the user feedback so far has been quite positive for the new get_model() method.

[NicolasHug]

I assume that the user feedback about get_model() came before get_model_builder() was exposed? It's still worth re-considering IMO.

There should be one-- and preferably only one --obvious way to do it.

@NicolasHug Just saw your message.

I'll try to accept that as true, but in all honesty, this is the exact same sequence of events as they happened in #6333. It makes me feel uncomfortable and I don't want to re-live that again. I'll disengage from this conversation.