Load model from model-zoo

[Zhou3983]

Instead of downloading the pre-trained weights manually, I try to load them from Huggingface model-zoo directly based on the document.

[achyudh]

Isn't #58 also trying to do the same thing with the hf-checkpoint flag?

In any case, the reason we didn't want to have the option to download a checkpoint in order to protect against any changes to these checkpoints by hugging-face, which would invalidate the results in our papers. Plus, we use Compute Canada to train most of our models and some of their servers don't have internet access.