Add pytorch dependency

[niksirbi]

PR #70 added a model which depends on pytorch, but the corresponding dependency hasn't been added.
We have to add torch to the dependencies, test for it, and update the corresponding installation instructions.

[adamltyson]

It's probably worth having a chat about model dependencies, there are two questions that come to mind:

1. How are dependencies installed, we we try to do everything with pip, put NeuralPlayground on conda-forge, or have custom installation instructions
2. Do we separate dependencies, or try to install everything? What if there are models with TF/JAX dependencies?

[niksirbi]

For now we've decided to take the following approach:

Don't include torch in the core pip dependencies. Instead, document it in the relevant TEM model and point users to Pytorch installation instructions. Perhaps we could even add some helpful warnings, if someone is trying to use the TEM model without torch installed.