Add Tensor Extension

[ljchang]

At some point we need to add an extension to accommodate tensors for multivariate rating data. Not sure how difficult it will be.

[ejolly]

There are some reasonably mature looking neural net recommender systems that may be a natural way to move forward on this. Just keep what I find here for future reference:

• Spotlight recommender system built in PyTorch
• Tensorflow Recommenders
  • Quick overview