Model downloads with S3

Download models with S3

• Improve speed and reliability by downloading models with S3

Misc fixes

• Use opencv headless to avoid GUI dependencies

What's Changed

• Add s3 loading functionality to from_pretrained by @tarun-menta in #320
• Swap to headless opencv by @VikParuchuri in #321

Full Changelog: v0.11.1...v0.12.0