Fix typing of the protocols.commutes() function (#5651)
Fix typing of the protocols.commutes() function (#5651)

Return bool when the `default` argument is not specified.
Otherwise allow return type to be the same as the `default` argument type.

Co-authored-by: Matthew Neeley <335c91217492d9eb9b627f3f9ed5c8b84eacbcea@google.com>
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq