Feature: L1, L2 Regularizers

[WuZhuoran]

We can implement Regularizer for each layer. The regularizers are applied on a per-layer basis. Here are 3 things that we can do:

1. Regularizer Class.
2. Layers class to use regularizer. (we can set a new parameter)
3. Regularizer Documentation.
4. Regularizer Test.

I will start a PR to solve 1 and 3. The rest part should be discussed before coding.

[WuZhuoran]

In order to avoid Keras or other package way of implementing this, we need to first discuss how to implement Regularizer in general. Then I will re commit everything.

[ddbourgin]

Closing due to inactivity