Clean up data streaming in topic coherence

[piskvorky]

Problem description

The topic coherence functionality seems to require the entire input corpus to be in RAM. This is bad because:

a) Unnecessary and wasteful: The input is only accessed as for doc in texts: so streamed iterables would work fine.
b) Not in line in Gensim's core interfaces, which stream data and expect iterables, precisely to be memory-efficient.

Steps/code/corpus to reproduce

See https://groups.google.com/g/gensim/c/tZ_qV5wsBDw for a user report.

OTOH, @gojomo reports another code path where it seems that iterables on input should work. So IDK. Investigate & clean up the docs to not require a "list of lists", at the very least. And if something doesn't support streaming of large data, it doesn't belong in Gensim.