Add return type annotation on MeasurementGate.full_invert_mask (#5080)

Review: @tanujkhattar 
Co-authored-by: Tanuj Khattar <[email protected]>

cirq-core/cirq/ops/measurement_gate.py

@@ -153,7 +153,7 @@ def with_bits_flipped(self, *bit_positions: int) -> 'MeasurementGate':
             self.num_qubits(), key=self.key, invert_mask=tuple(new_mask), qid_shape=self._qid_shape
         )
 
-    def full_invert_mask(self):
+    def full_invert_mask(self) -> Tuple[bool, ...]:
         """Returns the invert mask for all qubits.
 
         If the user supplies a partial invert_mask, this returns that mask