Add parametrization check to GlobalPhaseGate

Assume GlobalPhaseGate.coefficient is a numerical value otherwise.

Author: pavoljuhas

Diff for: cirq-core/cirq/ops/global_phase_op.py

@@ -13,19 +13,17 @@
 # limitations under the License.
 """A no-qubit global phase operation."""
 
-from typing import AbstractSet, Any, cast, Dict, Sequence, Tuple, TYPE_CHECKING, Union
+from typing import AbstractSet, Any, cast, Dict, Sequence, Tuple, Union
 
 import numpy as np
 import sympy
 
+import cirq
 from cirq import value, protocols
 from cirq._compat import deprecated_class
 from cirq.type_workarounds import NotImplementedType
 from cirq.ops import gate_operation, raw_types
 
-if TYPE_CHECKING:
-    import cirq
-
 
 @value.value_equality(approximate=True)
 @deprecated_class(deadline='v0.16', fix='Use cirq.global_phase_operation')
@@ -90,6 +88,7 @@ def _apply_unitary_(
     ) -> Union[np.ndarray, NotImplementedType]:
         if not self._has_unitary_():
             return NotImplemented
+        assert not cirq.is_parameterized(self)
         args.target_tensor *= cast(np.generic, self.coefficient)
         return args.target_tensor