GH-33: remove empty sentences

[alanakbik]

Users sometimes have problems with datasets that contain empty sentences (see #33). We currently issue a warning when an empty sentence gets constructed, but leave the users to filter empty sentences by themselves.

This PR adds a new function to the Corpus object, namely filter_empty_sentences(). By calling it, all empty sentences get removed.

Example:

# load dataset (IMDB dataset has no empty sentences)
corpus = IMDB().downsample(0.001)
print(corpus)

# add an empty sentence to the training split
corpus._train += SentenceDataset(Sentence(''))
print(corpus)

# call .filter_empty_sentences() to remove empty sentences
corpus.filter_empty_sentences()
print(corpus)

[kashif]

👍

[alanakbik]

👍