Add LiSHT activation function

[zaleslaw]

The LiSHT (Non-Parametric Linearly Scaled Hyperbolic Tangent) activation function is missed in KotlinDL.

Read more about LiSHT: Non-Parametric Linearly Scaled Hyperbolic Tangent Activation Function for Neural Networks

The desired PR addressing this issue should include:

• Implementation of activation class named as LishtActivation (you can take inspiration from the implementation of HardSigmoid as reference) added to the Activations.kt file
• Documentation of activation function
• JUnit tests in api module
• Support for export of activation function to JSON (see ModelSaver.kt)
• Support for import of activation function from JSON (see ModelLoader.kt)

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P.P.S Read the Contributing Guidelines.

The reference implementation could be taken from tensorflow-addons