Add warning when string is used as argument to infer_vector()

[tobycheese]

Another one that I just stumbled over.

infer_vector() in doc2vec.py requires a list of strings (words). If one does not pay attention and passes a string instead, this does not fail (as strings are lists of characters).

infer_vector() then calculates the vector based on the single characters in the string, which most of the time is not what one wants I think. Imho this merits a warning.

[piskvorky]

This is a welcome sanity-check, thanks!

@gojomo any reason the check wasn't previously included, do/did we have a reason to accept strings?

[menshikh-iv]

@tobycheese thanks for PR, please

• merge latest develop into your branch (I merged Minor corrections and improvements in Any*Vec docstrings #2345)
• raise a ValueError instead of warning if user pass string to infer_vector (instead of warning)

After, I'll merge your PR

[gojomo]

Only reason for no check previously was that parameter-name/doc-comment were thought clear enough, and perhaps if Python loose typing wants to let a string behave just like a list-of-one-character-strings, why should we complain? (It's not inconcievable that someone would be trying to create a model based on single-char tokens.) But certainly this bites people, and those who need single-char words can just force their strings to be lists, so some sort of warning or error is a good idea.

[piskvorky]

Thanks @tobycheese ! A small step for code, one giant leap for user-friendliness :)

[tobycheese]

Thanks for commiting, userfriendlyness ftw!