Fix signature of functions and methods in generated docs

[mkaze]

Summary

This PR resolves two issues:

1. The get_function_signature in autogen.py duplicates module name for non-method functions. For example, it returns keras.preprocessing.sequence.keras.preprocessing.sequence.pad_sequences instead of keras.preprocessing.sequence.pad_sequences.

2. Method functions and non-method functions should be distinguished from each other. Since, the module name of non-method functions should be included in the signature in the docs (like keras.preprocessing.sequence.pad_sequences(...)) whereas for the methods of classes it should not be included (like compile(...) for Sequential class). Therefore, a new 'methods' key is introduced to make this distinction.

Note that I temporary resolved the first issue by a merged PR (#10664); however that PR did not address the underlying problem.

Related Issues

#10658
#10662

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[Dref360]

LGTM thanks