Add sparse sum, mean and dot operator support

[kalyc]

Summary

Add sparse support for sum, mean and dot operators

Related Issues

Continuing - https://github.com/awslabs/keras-apache-mxnet/pull/159 Missing sparse operators

PR Overview

• [y] 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]

[kalyc]

Addressed comments. Testing on benchmark model not yet done.

[kalyc]

Addressed refactoring comments

[sandeep-krishnamurthy]

Thanks. LGTM. Going ahead with merging these operators. Please follow up with an end to end example using sparse tensor (may be think of having this end to end example work for your benchmarking as well)

[kalyc]

Yes will do - we will need to update the existing benchmark script to use these operators as well.