`PauliEvolutionGate` default label can be too long to serialize via qpy about qiskit

[jakelishman]

I think there's probably two parts to the solution here:

• PauliEvolutionGate absolutely should not be producing by default labels that use that many characters. It's not helpful to anybody, and I imagine it's slow to make as well - I agree we should definitely change that to something more sensible, arguably avoiding eagerly serialising the inner SparsePauliOp into a less efficient representation when the gate is created, even if we do partially truncate it - it seems very wasteful and prone to this sort of explosion. I don't use the gate, though, so I don't know what might be more useful.
• QPY should have an escape mechanism to avoid hard crashing - maybe we just emit a warning on too-long labels and truncate them at the maximum length. We could consider introducing a new QPY format version to support arbitrary-length labels in some form or another too, if it's necessary.

[Lancelot03]

hi, @garrison as by your suggestion we can implement the truncate function like this

def serialize(self):
        label = self._get_full_label()
        truncated_label = self._truncate_label(label)
            return truncated_label
    
def _truncate_label(self, label, max_length=65536):
        if len(label) <= max_length:
            return label
        else:
            index = max_length - 1
            while index >= 0 and label[index] != '+':
                index -= 1
            if index < 0:
                raise ValueError("Cannot truncate label without breaking terms")
            return label[:index] + ' {...}'

we can do some modification in the code according to the types of input.