[#5644] improve operator support for new assert_equal function: group 1

As suggested in [#5644], makes kernels _equals_operators, _equal_controlled
_equal_pow, _equal_adjoint and _equal_exp return either True or a string
explaining why two operators are not the same.

doc/releases/changelog-dev.md

@@ -276,6 +276,7 @@ Ahmed Darwish,
 Isaac De Vlugt,
 Diksha Dhawan,
 Pietropaolo Frisoni,
+Eugenio Gigante,
 Emiliano Godinez,
 David Ittah,
 Soran Jahangiri,

pennylane/ops/functions/equal.py

@@ -399,7 +399,7 @@ def _equal_controlled(op1: Controlled, op2: Controlled, **kwargs):
  if dict(zip(op1.control_wires, op1.control_values)) != dict(
  zip(op2.control_wires, op2.control_values)
  ):
- return f"op1 and op2 have different control wires/values. "
+ return "op1 and op2 have different control wires/values."
 
  if not qml.equal(op1.base, op2.base, **kwargs):
  return f"op1 and op2 have different bases because op1.base is not equal to op2.base. Got {op1.base} and {op2.base}."