Addons: Implement Sparsemax Activation and Loss

[seanpmorgan]

Per the RFC, we need to move sparsemax from contrib to addons:

• [X] Moving sparsemax to addons/activations and modifying test cases to run in TF2.
• [X] Restructuring sparsemax_loss in an OO format by inheriting from base Keras Loss, modifying the code to match those APIs, and modifying test cases to run in TF2.

https://arxiv.org/abs/1602.02068

[facaiy]

@AndreasMadsen Hi, Andreas. As the author of sparsemax, would you like to migrate it from contrib to addons? That would be greatly helpful :-)

[AndreasMadsen]

Yep. I will give it a go. Perhaps next week.

[facaiy]

Great!