Fix bug in is_sparse() method, enable sparse test in training

[kalyc]

Summary

Fix bug in is_sparse() method, enable sparse test in training Disable sparse concat test for testing CI build

Related Issues

Examples were failing locally - verified that they work locally now Unit tests were also failing locally under test_training - verified that they pass now

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]

[roywei]

closing and reopen to trigger CI

[kalyc]

Related issue - to enable sparse concat test - https://github.com/awslabs/keras-apache-mxnet/issues/179