*   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.
    *   Update `Fingerprint64Map` to use aliases
    *   Add legacy string flat hash map op kernels.

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