ObservablesArray docstring improvement

[gadial]

Summary

Expands the docstring of ObservablesArray.tolist to make it clear it might return a scalar in the case the observables array is of dimension 0.

Fixes #13402.

Details and comments

ObservablesArray.tolist is based on numpy's tolist method which returns a list with nesting level equal to the dimension of the numpy array being converted. When an observable array is initialized using a single observable (as opposed to a list containing a single observable) the internal representation of the array will be as a dimension 0 numpy array, resulting on a somewhat confusing return value:

>>> from qiskit.primitives.containers.observables_array import ObservablesArray
>>> oa = ObservablesArray.coerce(["Z"])
>>> print(type(oa.tolist()))
<class 'list'>
>>> oa = ObservablesArray.coerce("Z")
>>> print(type(oa.tolist()))
<class 'dict'>

This PR clarifies the issue in the method's docstring.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core
• @ajavadia
• @levbishop
• @t-imamichi

[coveralls]

Pull Request Test Coverage Report for Build 12646326577

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 814 unchanged lines in 57 files lost coverage.
• Overall coverage increased (+0.005%) to 88.94%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit/circuit/library/quantum_volume.py	1	93.18%
qiskit/primitives/statevector_sampler.py	1	99.07%
qiskit/primitives/base/base_estimator.py	1	96.97%
qiskit/converters/circuit_to_instruction.py	1	98.18%
qiskit/transpiler/passes/routing/commuting_2q_gate_routing/commuting_2q_gate_router.py	1	99.11%
qiskit/circuit/classical/types/ordering.py	1	98.31%
qiskit/pulse/instructions/directives.py	1	97.14%
crates/accelerate/src/unitary_synthesis.rs	1	92.2%
qiskit/pulse/transforms/alignments.py	1	96.27%
crates/accelerate/src/circuit_library/quantum_volume.rs	1	96.67%

Totals
Change from base Build 11784569909:	0.005%
Covered Lines:	79420
Relevant Lines:	89296

---

💛 - Coveralls

[ElePT]

Naive question, why is the output type a dict? I would expect a string.

[gadial]

In general, an observable can be a weighted sum of Paulis, so it's represented by a dictionary mapping the Pauli (or Pauli string) to its weight. See here.

[ihincks]

Oh I just realized on re-review that this return annotation also needs to be updated

[gadial]

Thanks, done in c968189