Update `QasmUGate` to support symbols in parameters.

[ArfatSalman]

Description of the issue
assert_circuits_with_terminal_measurements_are_equivalent throws an error when a QasmUGate uses a Symbol parameter.

How to reproduce the issue

import cirq
from sympy import Symbol
from cirq.circuits.qasm_output import QasmUGate
import numpy as np


qr = [cirq.NamedQubit("q" + str(i)) for i in range(5)]

p_96fafa = Symbol("p_96fafa")

circuit = cirq.Circuit()

circuit.append(
    QasmUGate(5.887184334931191 / np.pi, p_96fafa / np.pi, 1.4112277317699358 / np.pi)(
        qr[4]
    )
)


circuit = cirq.resolve_parameters(
    circuit,
    {
        "p_96fafa": 0.07157463504881167,
    },
)

cirq.testing.assert_circuits_with_terminal_measurements_are_equivalent(circuit, circuit)
# TypeError: cirq.apply_unitaries failed. There was a non-unitary value in the `unitary_values` list.

put long logs in details blocks like this

Cirq version
You can get the cirq version by printing cirq.__version__. From the command line:

1.1.0

[github-actions]

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[viathor]

This happens because cirq.resolve_parameters only performs simple symbol substitution, not full evaluation of symbolic expressions. The circuit is

q4: ───cirq.circuits.qasm_output.QasmUGate(theta=1.8739489755949428, phi=Mod(0.318309886183791*p_96fafa, 2), lmda=0.44920773867909736)───

both before and after the call to cirq.resolve_parameters. Unfortunately, QasmUGate manipulates the angles you pass in (by doing modulo), so replacing p_96fafa / np.pi with an instance of sympy.Symbol will not help.

I think what needs to be done here is:

• At absolute minimum: we must add type annotations for QasmUGate to make it clear that symbolic values are unsupported.
• Better: fix QasmUGate to support symbolic parameters.

We could also revisit the question of performing some symbolic evaluations in cirq.resolve_parameters.

[viathor]

Did a quick search to see whether we ourselves pass symbolic values into QasmUGate and it turns out we do, so it's not as simple as adding type annotations. For example, we use QasmUGate to implement PhasedXZGate._qasm_.