Fix Moment.resolve_parameter for constant sympy expressions (#6794)

Problem: `Moment._resolve_parameters_` might keep constant sympy expression
if its resolved value is numerically equal.

Solution: Check if parameterization changed for the resolved operation and
if so replace the original operation with resolved (even if equal).

Fixes #6778

---------

Co-authored-by: Pavol Juhas <[email protected]>
Co-authored-by: Noureldin <[email protected]>

cirq-core/cirq/circuits/moment.py

@@ -275,8 +275,11 @@ def _resolve_parameters_(
         resolved_ops: List['cirq.Operation'] = []
         for op in self:
             resolved_op = protocols.resolve_parameters(op, resolver, recursive)
-            if resolved_op != op:
-                changed = True
+            changed = (
+                changed
+                or resolved_op != op
+                or (protocols.is_parameterized(op) and not protocols.is_parameterized(resolved_op))
+            )
             resolved_ops.append(resolved_op)
         if not changed:
             return self

cirq-core/cirq/circuits/moment_test.py

@@ -294,6 +294,14 @@ def test_resolve_parameters():
     moment = cirq.Moment(cirq.X(a) ** sympy.Symbol('v'), cirq.Y(b) ** sympy.Symbol('w'))
     resolved_moment = cirq.resolve_parameters(moment, cirq.ParamResolver({'v': 0.1, 'w': 0.2}))
     assert resolved_moment == cirq.Moment(cirq.X(a) ** 0.1, cirq.Y(b) ** 0.2)
+    # sympy constant is resolved to a Python number
+    moment = cirq.Moment(cirq.Rz(rads=sympy.pi).on(a))
+    resolved_moment = cirq.resolve_parameters(moment, {'pi': np.pi})
+    assert resolved_moment == cirq.Moment(cirq.Rz(rads=np.pi).on(a))
+    resolved_gate = resolved_moment.operations[0].gate
+    assert not isinstance(resolved_gate.exponent, sympy.Basic)
+    assert isinstance(resolved_gate.exponent, float)
+    assert not cirq.is_parameterized(resolved_moment)
 
 
 def test_resolve_parameters_no_change():