[FEATURE] Isolated version of models about darts

[madtoinou]

Hi @marcelo-bn,

If you want to use a Darts is a lightweight environment, you can use the onnx format (check the code snippet at the end of issue #553). However, you will be responsible for reshaping/preparing the samples for inference and won't have access to the TimeSeries data-structure anymore.

Maintaining a separated version of the library for each model would actually not solve the problem since a lot of dependencies would remain (and it would be not realistically maintainable).

Feel free to fork Darts and remove all the features/models you don't need, hopefully this will reduce the requirements enough so that you can use our TFT implementation with your current constraints.