Local checkpoint support

[tymanab]

Hi,
Thanks for release this awesome project. Could you please add an example about how to load a clip upsampler from pre-download local checkpoint? I don't have stable network connection to torch hub, and torch.hub.load() will not work sometimes even I already cached the model.

Besides，is there any method to make this model work with OpenAI‘s original CLIP implementions？A special version of CLIP will bring a lot of challenges for models that used clip backbone encoder.