*   `regression_head` now accepts customized link function, to satisfy the
        usage that user can define their own link function if the
        `array_ops.identity` does not meet the requirement.
    *   Fix `initialized_value` and `initial_value` behaviors for
        `ResourceVariables` created from `VariableDef` protos.
    *   Add TensorSpec to represent the specification of Tensors.