This symbol is a type alias.
Source:
VALIDATOR_TYPE = Callable[
Sequence[cirq.circuits.circuit.AbstractCircuit],
Sequence[Union[ForwardRef('cirq.ParamResolver'), Mapping[ForwardRef('cirq.TParamKey'), ForwardRef('cirq.TParamValComplex')], NoneType, cirq.study.sweeps.Sweep, cirq.study.sweepable._Sweepable]],
Union[int, Sequence[int]],
NoneType
]