add an ImportError to tell the user that datasets must be install to fit a model

[h4c5]

Minor change to add an ImportError when datasets is not installed and the user try to fit a model ( Closes #3019 )

[tomaarsen]

Well spotted! Apologies for the slight delay.
I hadn't noticed this myself as model.fit is the old way of finetuning a model, nowadays the Trainer is preferred: https://sbert.net/docs/sentence_transformer/training_overview.html

I do still want to merge this though - it'll help those who still want to use the old approach or still have scripts for model.fit.

• Tom Aarsen