Sanity check for hs and negative in Word2Vec

[gau-nernst]

Fixes #1983

Changes:

• Raise ValueError when both hs=0 and negative=0
• Raise warning when hs and negative are nonzero
• Add test for the above
• Update description about hs and negative to avoid confusion

@gojomo Please let me know if you want any changes. Cheers!

[gojomo]

Looks very good to me!

[piskvorky]

Many thanks! Let's wait for @mpenkov to review and merge.

By the way, could other models in Gensim (doc2vec) benefit from a similar cleanup?

[gau-nernst]

Many thanks! Let's wait for @mpenkov to review and merge.

By the way, could other models in Gensim (doc2vec) benefit from a similar cleanup?

It seems like doc2vec and fasttext in gensim have the same problem and would benefit from the same fix. I can add the changes to doc2vec and fasttext also, once you confirm it!

[piskvorky]

Yes please!

[mpenkov]

LGTM!

[mpenkov]

Merging. Let's handle the other models in a separate PR, to keep the ball rolling.

[gau-nernst]

Merging. Let's handle the other models in a separate PR, to keep the ball rolling.

I did a quick check. Since FastText and Doc2Vec inherit Word2Vec class, they automatically have the sanity check for hs and negative after this commit. We can add tests for FastText and Doc2Vec but I think it is not so necessary.