Add tf.diagPart() modeled after [tf.diag_part()](https://www.tensorflow.org/api_docs/python/tf/diag_part) in Python. See [discussion](https://groups.google.com/a/tensorflow.org/forum/?utm_medium=email&utm_source=footer#!msg/tfjs/QOzq60A3n48/ZiaEcGLNEwAJ) for context.
