GH-1509: split datasets into smaller submodules

[alanakbik]

Closes #1509

This PR splits the datasets.py into several submodules organized in a datasets/ folder. The submodules are:

• sequence_labeling.py for all sequence labeling datasets
• document_classification.py for all document classification datasets
• treebanks.py for all dependency parsed corpora (UD treebanks)
• text_text.py for all bi-text datasets (currently only parallel corpora)
• text_image.py for all paired text-image datasets (currently only Feidegger)
• base.py for remaining basic classes

All datasets are still exposed through the datasets package, so it is still possible to load corpora with

from flair.datasets import TREC_6

without specifying the submodule.