Implement GroupNorm, LayerNorm & InstanceNorm

[seanpmorgan]

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

This will involve restructuring in an OO format by inheriting from base Keras Layer, modifying the code to match those APIs, and modifying test cases to run in TF2.

https://arxiv.org/abs/1607.06450

EDIT: It will make more sense to implement GroupNorm, and then created interfaces for LayerNorm and InstanceNorm

Group Norm: https://arxiv.org/pdf/1803.08494.pdf

[lgeiger]

What do you think about implementing this together with #7 using a GroupNormalization Layer which is a generalized case of layer normalization and instance normalization?

[seanpmorgan]

@lgeiger Thanks for the suggestion! This would be much more succinct.

I would vote to have interfaces named LayerNorm and InstanceNorm which are GroupNorm with 1 group and input_dim# groups respectively.

[Smokrow]

I will give this a try