Implement SE3 Transformers

[dmadisetti]

Hi, I'm really excited for this library. I think some of the scalability tricks you have promised under the hood look exceptionally nice.

Building off of the GATv2 example, I'd really like to implement SE3 Transformers (https://arxiv.org/pdf/2006.10503.pdf)

I understand you have a roadmap, and this is an alpha, but I haven't found a SE3 implementation in tensorflow I like and this seems the most promising starting point.

I am able to compile and run tests (from my own branch). I can either try and hack something together for myself, or potentially layout a spec with someone from the team to do this.