Support model download in pip install phase

[jschennings]

Hello,

I'm currently looking into implementing this package into a microservice but run into problems. The issue I have is that I don't want to allow the kubernetes pods to download additional packages / etc for safety reasons.

Therefore I would like the possibility to "pre download" the weights during the pip install phase, like pip install rembg[u2net] or similar to not have to call download_models in runtime.

Thanks in advance!