Add OBPEstimator

[samanthavbarron]

This PR adds an Estimator interface to OBP. This allows the user to compute expectation values using qiskit_addon_obp.backpropagate. This interface allows for arbitrarily shaped EstimatorPubs, handles parameter bindings, etc.

There are likely some remaining questions before this should be merged if at all, but IMO an Estimator interface of some sort is a natural addition.

For example, currently: if the remaining circuit after backpropagation is non-empty, the OBPEstimator will raise a NotImplementedError. I left a comment with a potential suggestion for how to handle this case:

            # In this case, one alternative is to have the user supply an auxiliary estimator
            # The auxiliary estimator would compute the expectation value wrt the remaining
            # slices. E.g. The primary use case would likely be providing an estimator which
            # uses an actual backend to compute the remaining expectation value, etc.

Usage

import numpy as np
from qiskit import QuantumCircuit
from qiskit.primitives.containers.estimator_pub import EstimatorPub
from qiskit.primitives.containers.observables_array import ObservablesArray
from qiskit.quantum_info import SparsePauliOp
from qiskit_addon_utils.slicing import slice_by_barriers

from qiskit_addon_obp.obp_estimator import OBPEstimator

# Make your EstimatorPub

theta = np.pi / 6
qc = QuantumCircuit(1)
qc.rz(theta, 0)
qc.barrier()
qc.ry(theta, 0)
obs = SparsePauliOp("X")
pub = EstimatorPub(qc, ObservablesArray([obs]))

# Compute EVs with OBP
OBPEstimator(slicer=slice_by_barriers).run([pub]).result()[0].data.evs

array([0.5+0.j])

[coveralls]

Pull Request Test Coverage Report for Build 15192597933

Details

• 22 of 65 (33.85%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-7.8%) to 92.238%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_addon_obp/utils/broadcast_indices_iter.py	2	11	18.18%
qiskit_addon_obp/obp_estimator.py	20	54	37.04%

Totals
Change from base Build 15172425156:	-7.8%
Covered Lines:	511
Relevant Lines:	554

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💛 - Coveralls