Cannot pass a list of Pauli words to `observe_async`

[bmhowe23]

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Describe the bug

Passing a list of Pauli words to a cudaq.observe_async() function produces the following error:

RuntimeError: Unable to cast Python instance of type <class 'str'> to C++ type 'cudaq::pauli_word'

It works fine if you pass to cudaq.observe().

Steps to reproduce the bug

You can reproduce the bug by create a file called test.py with the following:

import cudaq
import numpy as np
import random

random.seed(13)
np.random.seed(13)

def generateRandomPauliStrings(numQubits, numPaulis):
    s = ['X', 'Y', 'Z', 'I']
    return [
        ''.join([random.choice(s)
                for i in range(numQubits)])
        for i in range(numPaulis)
    ]

def build_cudaq_obs(hs, paulis):
    observable = cudaq.SpinOperator()
    for h, p in zip(hs, paulis):
        observable += h * cudaq.SpinOperator.from_word(p)
    return observable - cudaq.SpinOperator()


@cudaq.kernel
def gqeCirc(N: int, thetas: list[float], paulis: list[cudaq.pauli_word]):
    q = cudaq.qvector(N)
    for i in range(len(paulis)):
        exp_pauli(thetas[i], q, paulis[i])

numQubits = 4
numPaulis = 8
numberOfTerms = 4

# Generate the observable
obs_ps = generateRandomPauliStrings(numQubits, numberOfTerms)
obs = build_cudaq_obs([1.0]*len(obs_ps), obs_ps)

pauliStings = generateRandomPauliStrings(numQubits, numPaulis)
ts = np.random.rand(len(pauliStings))

exp_val = cudaq.observe_async(gqeCirc, obs, numQubits, list(ts), pauliStings).get().expectation()
print(exp_val)

When run, it produces this error:

$ python3 test2.py 
RuntimeError: Unable to cast Python instance of type <class 'str'> to C++ type 'cudaq::pauli_word'

Expected behavior

It should run without errors

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA Quantum version: latest (bf57970)
• Python version:
• C++ compiler:
• Operating system:

Suggestions

PR forthcoming