Add sparse inference & training benchmark

[kalyc]

Summary

Add inference benchmark for sparse

Related Issues

PR Overview

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

[kalyc]

PR continued here - https://github.com/awslabs/keras-apache-mxnet/pull/197

Note - We have already pushed the benchmark scripts