Add docstring for each Python module

[nudles]

1. The module's docstring should explain the classes and functions in this module.
2. Example usages should be given.
3. How to add a new sub-class should be explained, e.g., how to add a new Layer subclass in layer.py, how to add a new Operator subclass in autograd.py.

The docstrings will be used to build the API documentation pages at https://apache-singa.readthedocs.io/en/latest/