losses.
    *   Added `LinearOperator.adjoint` and `LinearOperator.H` (alias).
    *   Expose CriticalSection in core as `tf.CriticalSection`.
    *   Enhanced graphviz output.
    *   Add opkernel templates for common table operations.
    *   Fix callbacks do not log values in eager mode when a deferred build
        model is used.
    *   `SignatureDef` util functions have been deprecated.