GH-748: add more classification datasets

[alanakbik]

This PR adds more datasets to the new flair.datasets way of loading data. It adds:

• IMDB dataset
• TREC_50 dataset

It also adds the popular 20newsgroups classification dataset to Flair.

Load with:

import flair.datasets

# load IMDB dataset
imdb_corpus = flair.datasets.IMDB()
print(imdb_corpus)

# load 20 newsgroups dataset
newsgroup_corpus = flair.datasets.NEWSGROUPS()
print(newsgroup_corpus) 

[alanakbik]

👍

[kashif]

👍