Add HardShrink activation function

[zaleslaw]

The Hard shrink activation function is missed in KotlinDL.

The desired PR addressing this issue should include:

• Implementation of activation class named as HardShrinkActivation (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)

P.S. If you want to take this ticket, please leave the comment below
P.P.S Read the Contributing Guidelines.

The reference implementation could be taken from tensorflow-addons

[therealansh]

Hey @zaleslaw , i can take this one up to complete 0.3 milestones quick. But does Ops provide any comparison function since it requires lower and upper value to be compared?

[zaleslaw]

Hmm, you right about upper/lower values. Could it be implemented via composition of tf.maximum(tf.minimum(input, lower), upper)?