*   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`.
    *   `tf.io.parse_sequence_example` and `tf.io.parse_single_sequence_example`
        now support ragged features.
    *   Fix `while_v2` with variables in custom gradient.