Support models that can input and predict multidimensional tensors

[MischaPanch]

This issue is only about setting up models and training, evaluation of such models will be handled in a separate issue. Thus, it is mainly about writing the TensorModel abstraction and some implementations of it.

The goal is to enable training of autoencoders, prediction of 2-dimensional tensors for geospatial analysis and so on.

I am not entirely sure how to deal with models that take multi-dim data and predict scalars like CNN-based classifiers and regressors (a very common use case). VectorModel does not feel like the right fit, nor does TensorModel (because it will focus on tensor-like prediction). @opcode81 Do you have an idea about that?

[MischaPanch]

Done