Allow cirq.resolve_parameter compatible with t-units

[BichengYing]

Is your feature request related to a use case or problem? Please explain

Minimal example:

import cirq
import cirq_google as cg
import tunits as tu
import sympy

q = cirq.q(0, 0)
c = cirq.Circuit(cirq.X(q)** sympy.Symbol("a"))
# !!!!! I know this is not a valid usage case. The point is just for showing the error.!!!
cirq.resolve_parameters(c, {"a": 10*tu.ns}) 

We will get the following error see that line f = lambda x: x if symbolic(x) else float(x)

File ~/.virtualenvs/pyle3/lib/python3.12/site-packages/cirq/ops/eigen_gate.py:316, in EigenGate._value_equality_values_.<locals>.<lambda>(x)
    307 """The phases by which we multiply the eigenspaces.
    308 
    309 The default implementation assumes that the eigenspaces are constant
   (...)
    313 fields that affect the eigenspaces.
    314 """
    315 symbolic = lambda x: isinstance(x, sympy.Expr) and x.free_symbols
--> 316 f = lambda x: x if symbolic(x) else float(x)
    317 shifts = (f(self._exponent) * f(self._global_shift + e) for e in self._eigen_shifts())
    318 return tuple(s if symbolic(s) else value.PeriodicValue(f(s), 2) for s in shifts)

File ~/.virtualenvs/pyle3/lib/python3.12/site-packages/tunits/core/cython/with_unit.pyx:399, in tunits_core.WithUnit.__float__()

UnitMismatchError: '10 ns' can't be stripped into a float; not dimensionless. 

Describe the solution you would prefer

Is that possible that we can avoid this casting to float and allow tunit.Value as another possible case? This can unblock lots of internal gate usages.

How urgent is this for you? Is it blocking important work?

P2 – we should do it in the next couple of quarters

[maffoo]

In the example you gave, the problem is not with cirq.resolve_parameters, rather the issue is that EigenGate can't be exponentiated with a time (just doing X**(10 * tu.ns) with no paramount resolution will show the same issue). Do you have a case where a tunits value is allowed if used directly, but fails with parameter resolution?

[BichengYing]

Yeah, that is not a good example. Let's use the wait gate as example:

Cirq/cirq-google/cirq_google/ops/wait_gate.py

Lines 60 to 66 in 17c4e95

	def _resolve_parameters_(
	self, resolver: cirq.ParamResolver, recursive: bool
	) -> WaitGateWithUnit:
	if isinstance(self._duration, sympy.Symbol):
	_duration = su.direct_symbol_replacement(self._duration, resolver)
	return WaitGateWithUnit(_duration, qid_shape=self._qid_shape)
	return self

Let we replace the line 64 as _duration = cirq.resolve_parameters(self._duration, resolver) instead, running the test, we will encounter the following error:

    def test_wait_gate_with_units_resolving() -> None:
        gate = wg.WaitGateWithUnit(sympy.Symbol("d"))
    
>       resolved_gate = cirq.resolve_parameters(gate, {"d": 10 * tu.ns})
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

cirq-google/cirq_google/ops/wait_gate_test.py:45: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/cirq/protocols/resolve_parameters.py:190: in resolve_parameters
    result = getter(param_resolver, recursive)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cirq-google/cirq_google/ops/wait_gate.py:64: in _resolve_parameters_
    _duration = cirq.resolve_parameters(self._duration, resolver)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/cirq/protocols/resolve_parameters.py:176: in resolve_parameters
    return cast(T, param_resolver.value_of(val, recursive))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/cirq/study/resolver.py:133: in value_of
    v = _resolve_value(param_value)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/cirq/study/resolver.py:288: in _resolve_value
    if val == sympy.pi:
       ^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/sympy/core/numbers.py:3458: in __eq__
    other = _sympify(other)
            ^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/sympy/core/sympify.py:505: in _sympify
    return sympify(a, strict=True)
           ^^^^^^^^^^^^^^^^^^^^^^^
../.virtualenvs/cirq-py3/lib/python3.12/site-packages/sympy/core/sympify.py:428: in sympify
    return sympify(coerce(a))
                   ^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

>   ???
E   tunits_core.UnitMismatchError: '10 ns' can't be stripped into a float; not dimensionless.

tunits/core/cython/with_unit.pyx:399: UnitMismatchError

[dstrain115]

One potential issue is that we would need to add tunits as a dependency for cirq-core. I am reluctant to allow more dependencies that are largely Google-specific to cirq-core.

[dstrain115]

Discussed in cirq cync: we do not want to add this dependency to cirq-core if we can avoid it, since this will include compiled code and cython dependencies for all cirq users. If there is a way to duck-type it without importing, we could consider a change to cirq-core.