Introduce tf.contrib.control_flow.new_cond.

new_cond is a new implementation of tf.cond. Instead of emitting
control flow ops (i.e. Switch and Merge nodes), new_cond emits a
single If op, which represents the conditional branches as TF
functions.

With this change, users can use new_cond and take its gradient.

The idea is for new_cond to eventually replace tf.cond. There are
several functional and performance gaps that must be addressed first,
including:
* Gradients won't work on imported graphs
* Misc. limitations of TF functions (lack of collections, device scopes, etc.)
PiperOrigin-RevId: 199346735