Add Mish activation function

[zaleslaw]

The Mish activation function is missed in KotlinDL.

Read more about Mish: A Self Regularized Non-Monotonic Neural Activation Function

The desired PR addressing this issue should include:

• Implementation of activation class named as MishActivation (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

[zaleslaw]

Hi, @digantamisra98! Please leave a comment here to assign this ticket to you.
As I understand, you are the author of the Mish paper, a really great paper and results!

[digantamisra98]

@zaleslaw Yes, I am taking this up and will be happy to add others over the weekend (hopefully). Thank you for your compliment and willingness to add it in the library :)

[zaleslaw]

@digantamisra98 I hope that KotlinDL could be more open to DL innovations than Keras with huge codebase and be faster in implementation of new ideas, will wait review request from you.

[digantamisra98]

The PR is ready for review (I don't know if I can like request a review on the PR itself unless I'm a collaborator with write access).