Deal with not-needed imports and requirements

[abigailgold]

At the moment due to the project structure all requirements need to be present even to use just some of the toolkit's functionality. This is especially annoying for the underlying ML frameworks, which all need to be installed just to use one of them. Need to rearrange imports such that unused frameworks are not required.

[abigailgold]

Also should have separate requirements files per module (as dependencies and versions may vary).
https://www.packetcoders.io/pip-trick-for-splitting-dev-requirements/