download_pretrained for demo API does not cache downloaded files/models

[jonheng]

To allow the containers to start up quicker, models and files were downloaded and cached during build time.

Recent changes in the huggingface transformers package has broken this functionality:

• Released in v4.22.0
• Issue

Possible choices moving forward:

• Write a simple caching utility function
• Stick to versions of transformers before 4.22.0