Add lazy module loading

[mariosasko]

Discussed this change with @mttk on Slack.

Features:

• lazy module loading
• makes it possible to import DiskBackedDataset, HFDatasetConverter and YAKE from the top level __init__.py but only if the required dependencies are installed (pyarrow for DiskBackedDatasets, datasets for HFDatasetConverter, yake for YAKE).

[ivansmokovic]

LGTM. Good idea.

[mttk]

So, from what I understand, every exposed module would be importable by doing from podium import <module>.
In some cases, I would prefer if we maintained different submodules, for cases of implemented datasets (from podium.datasets import SST, IMDB) or preprocessing tools, etc.

How difficult/ugly do you think it would be to maintain some degree of import hierarchy?

[FilipBolt]

I like this change since it is sort of more in line what I see from more recent frameworks: flair, huggingface. I know a bit older frameworks like torch, numpy, or scikit-learn still have hierarchy, but if there is no performance difference I don't see much value in keeping imports separate.

[mariosasko]

@mttk from podium.datasets import SST, IMDB still works just fine. The import hierarchy is left untouched. I understand what you are saying, but I think the hierarchy is irrelevant to the user and by exposing the public componets in the top level __init__.py , it's easier for the user to reach them.