TorchANI 2.1

Edit: This release is not in PyPI because it exceeds the maximum file size limit of PyPI. We will make a new release 2.1.1 to remove models outside TorchANI. Models will be automatically downloaded when used for the first time

Highlights:

• TorchANI paper is submitted to JCIM (#465, #469)
• ANI2x model is added as a built-in model for inference. (#480)

Other changes:

• Switch to PyTorch implementation of AdamW (#464)
• Data API improvements (#463, #475)
• BuiltinNet is refactored (#474, #473, #476)
• SpeciesConverter device fix (#461, #462)
• Documentation improvements (#479, #478)
• Fix flake8 (#466)