Remove on_batch_begin and on_batch_end callbacks

[mpenkov]

Fix #2182

I added a documentation note to avoid using these callbacks. With Python, we cannot prevent people from implementing those callback handlers in their own subclasses, so a documentation warning is the most that we can do.

[piskvorky]

Looks good, thanks. But tests are failing + needs a mention in the Migration notes too.

[mpenkov]

The tests are failing for an unrelated reason.

504  Gateway Time-out [IP: 34.223.170.36 443]
Reading package lists...
E: The repository 'https://dl.bintray.com/sbt/debian  Release' is no longer signed.

Not sure what's going on there. If it doesn't self-resolve, I'll have to investigate explicitly later.

Updated the migration docs here.

[mpenkov]

Alright, I managed to work around the unrelated test failures. Please have a look at the PR and the migration note updates and let me know if this is good to merge.

[gojomo]

Looks good, though if progress_queue.callbacks now never consulted, it also need not be set (in "messy way") earlier:

https://github.com/RaRe-Technologies/gensim/blob/1efa5066ccd8c3e60898a4a7d64828c348b7a3fb/gensim/models/word2vec.py#L1406