Running vqe runtime program with aux operator fails with serialization error

[jyu00]

Information

• Qiskit IBMQ Provider version:
• Python version:
• Operating system:

What is the current behavior?

Running the vqe runtime program with aux operator would fail with

raise ValueError("Object arrays cannot be saved when "\n2021-09-09T21:39:52.232793489Z ValueError: Object arrays cannot be saved when allow_pickle=False\n'

Steps to reproduce the problem

Run this tutorial:
https://github.com/Qiskit/qiskit-nature/blob/stable/0.2/docs/tutorials/07_leveraging_qiskit_runtime.ipynb

What is the expected behavior?

The program should work

Suggested solutions

RuntimeEncoder currently use np.save() to save numpy arrays, since it's more efficient than tolist(). save(), however, doesn't support arrays with object types (e.g. dtype=object) when allow_pickle=False is set (we can't use pickle due to security). We can potentially use tolist() if save() doesn't work.