Add Gelu activation function

[zaleslaw]

The Gelu activation function is missed in KotlinDL.

The desired PR addressing this issue should include:

• Implementation of activation class named as GeluActivation (you can take inspiration from the implementation of HardSigmoid as reference) added to the Activations.kt file
• Add an approximate version of Gelu
• 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.

Read more about Gaussian Error Linear Units (GELUs)
and BERT: Pre-training of Deep Bidirectional Transformers for Language Understanding

The reference implementation could be taken from tensorflow-addons

[therealansh]

@zaleslaw I'll start working on this next up.