compiler produces compilation errors.
    *   TPUs can now be re-initialized multiple times, using
        `tf.tpu.experimental.initialize_tpu_system`.
    *   Add `RaggedTensor.merge_dims()`.
    *   Added new `uniform_row_length` row-partitioning tensor to
        `RaggedTensor`.
    *   Add `shape` arg to `RaggedTensor.to_tensor`; Improve speed of
        `RaggedTensor.to_tensor`.