Added Mish Activation Function

[digantamisra98]

Mish is a new novel activation function proposed in - https://arxiv.org/abs/1908.08681
It has shown promising results so far and has been adopted in several packages including:

TensorFlow Addons
FastAI-dev
SpaCy
Thinc
DarkNet
Yolo V3

All benchmarks, analysis and links to official package implementations can be found in this repository - https://github.com/digantamisra98/Mish

It would be nice to have Mish as an option within the activation function group.

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[digantamisra98]

@googlebot I signed it!

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[jekbradbury]

Thanks, Diganta. We're currently trying to keep jax.nn very minimal, and have tended to limit it to common functionality extracted from existing JAX neural net libraries. As a result, I'm unfortunately going to decline this PR. Sorry about that!

[digantamisra98]

Understandable. No worries.