restrict identity check to other.gate

Author: NoureldinYosri

cirq-core/cirq/ops/linear_combinations.py

@@ -570,10 +570,12 @@ def __iadd__(self, other):
         return self
 
     def __add__(self, other):
-        if isinstance(other, (gate_operation.GateOperation)):
-            other = PauliString(other)
         if not isinstance(other, (numbers.Complex, PauliString, PauliSum)):
-            return NotImplemented
+            try:
+                if isinstance(other.gate, identity.IdentityGate):
+                    other = PauliString(other)
+            except:
+                return NotImplemented
         result = self.copy()
         result += other
         return result