Created by: wangyang59

1. Exposed gather/expand/pad ops to python
2. Made variable name consistent between scatter and gather. I chose to use index instead of ids because lots of other files used index like scatter.h and gather.h.
3. Removed the check of type consistency in layer_function_generator.py so that gather and scatter ops can be called directly since the type of Input Index is integer and X can be float.
4. Corrected the documentation/comment in scatter op. It should be an update/replace operation instead of an add operation.