Allow symbol for coupling_mhz in CouplerPulse gate

[maffoo]

This is something we use on our internal version of this gate.

[vtomole]

If you want to allow the passing of symbolic expressions, you won't be able to evaluate the repr

gate = coupler_pulse.CouplerPulse(
        hold_time=cirq.Duration(nanos=10), coupling_mhz=sympy.cos(sympy.Symbol('x')) + 1 , rise_time=cirq.Duration(nanos=18)
    )
    cirq.testing.assert_implements_consistent_protocols(
        gate,
        setup_code='import cirq\nimport numpy as np\nimport sympy\nimport cirq_google',
        qubit_count=2,
        ignore_decompose_to_default_gateset=True,
    )

Fails with a NameError("name 'cos' is not defined").

The resolver doesn't seem to work with this change

circuit = cirq.Circuit(gate.on(cirq.LineQubit(0), cirq.LineQubit(1)))
print(cirq.resolve_parameters(circuit, {"x": 0.1}))
# prints
# 0: ───/‾‾(10 ns@cos(x) + 1MHz)‾‾\───
#       │
# 1: ───/‾‾(10 ns@cos(x) + 1MHz)‾‾\───

[maffoo]

Thanks for taking a look, @vtomole. I don't think repr evaluate is particularly important and I presume the same thing would fail for other parameterized gates as well. I did add support for parameter resolution and related protocols (parameter_names and is_parameterized). PTAL

[vtomole]

I don't think repr evaluate is particularly important

Not important, but should be as simple as modifying CouplerPulse's repr to use proper_repr

from cirq._compat import proper_repr
def __repr__(self) -> str:
        return (
            'cirq_google.experimental.ops.coupler_pulse.'
            + f'CouplerPulse(hold_time={self.hold_time!r}, '
            + f'coupling_mhz={proper_repr(self.coupling_mhz)}, '
            + f'rise_time={self.rise_time!r}, '
            + f'padding_time={self.padding_time!r})'
        )

[vtomole]

Nit: For consistency with the __init__ params

Suggested change

	cirq.is_parameterized(self.hold_time)
	or cirq.is_parameterized(self.rise_time)
	or cirq.is_parameterized(self.padding_time)
	or cirq.is_parameterized(self.coupling_mhz)
	cirq.is_parameterized(self.hold_time)
	or cirq.is_parameterized(self.coupling_mhz)
	or cirq.is_parameterized(self.rise_time)
	or cirq.is_parameterized(self.padding_time)

[maffoo]

Done.

[vtomole]

LGTM