Add IndRNN

[Carlos Martin]

Feature request: Add IndRNNCell to flax's recurrent cells.

• Independently Recurrent Neural Network (IndRNN): Building a Longer and Deeper RNN. CVPR 2018.

It currently has 1004 citations according to Google Scholar.

I can submit a PR for this.

[Cristian Garcia]

One big factor against adding IndRNN to Flax is that neither Pytorch nor Keras provides this layer.