add indrnn implementation

[Aphlios]

This pull request introduces a new implementation of the Independently Recurrent Neural Network (IndRNN) cell and integrates it into the model selection utilities. The main changes include adding the IndRNN cell and its supporting classes, registering it in the training utilities, and making it available for model selection.

[smorad]

Thanks for the PR. I have a few questions:

• Where does recurrent_min_abs with a default of 0 come from? Is this from the original paper?
• Is the special initialization also in the original paper?

I would like to keep the models as simple as possible. If these are not listed in the original paper we should not implement them.