Fix docstrings for gensim.models.AuthorTopicModel (#1907)

* Add docustrings for Author-topic model

The PR aims to fix the docstrings for Author-topic model.

* Make PEP8 fixes

* Add docstrings for rest of the functions.

* Make updates to the docstrings

* Make fixes to docstrings

* Update atmodel.py

* Make update to example

* Make final edits

* Update atmodel.py

* Update atmodel.py

* fix PEP8

* fix atmodel[1]

* fix atmodel[2]

* fix atmodel[3]

* fix atmodel[4]