Rust Implementation for using SparseObservable in sampled_expectation_value

[velocityraptor7085]

Summary

• Uses a rust-based implementation for more efficiently computing expectation values for SparseObservable.
• Fixes sampled_expectation_value does not accept SparseObservable #13446
• Based on the suggestion by @mtreinish in the Python-based solution (refer to Added support for using SparseObservable in sampled_expectation_value #14502).
• Has a faster average execution time of 0.000013 seconds compared to the earlier approach (based on the same benchmarking used in the original pull request).

Details and comments

The changes made in the rust-based implementation include:

1. Adding a dependency on the qiskit-quantum-info crate the Cargo.toml file within the qiskit-accelerate crate.
2. Adding a rust function (sampled_expval_sparse_observable) to extract the operator strings and coefficients (instead of in Python), and pass them into the existing rust routines for expectation value computations (in crates/accelerate/src/sampled_exp_val.rs).
3. Makes the inner trait of the PySparseObservable wrapper (in crates/quantum_info/src/sparse_observable/mod.rs) as pub (This was done since passing a PyRef<SparseObservable needed a PyClass implementation, but adding #[pyclass] to the SparseObservable rust struct causes conflicting implementations due to the wrapper's implementation. I am unsure if there is a more elegant / desirable way to do this and would appreciate any comments on the same).
4. Calls the rust routine from the sampled_expval.py in an additional elif clause.

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[coveralls]

Pull Request Test Coverage Report for Build 17955752966

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 52 of 59 (88.14%) changed or added relevant lines in 3 files are covered.
• 1240 unchanged lines in 29 files lost coverage.
• Overall coverage decreased (-0.07%) to 88.307%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/accelerate/src/sampled_exp_val.rs	44	51	86.27%

Files with Coverage Reduction	New Missed Lines	%
crates/cext/src/transpiler/passes/split_2q_unitaries.rs	1	90.63%
crates/cext/src/transpiler/passes/vf2.rs	1	82.61%
crates/circuit/src/parameter/parameter_expression.rs	1	82.79%
crates/qasm2/src/expr.rs	1	93.63%
crates/quantum_info/src/sparse_observable/mod.rs	1	94.04%
crates/qasm2/src/lex.rs	2	92.27%
qiskit/transpiler/passes/layout/sabre_pre_layout.py	2	96.55%
crates/cext/src/transpiler/passes/unitary_synthesis.rs	3	96.67%
crates/quantum_info/src/unitary_sim.rs	4	85.11%
crates/quantum_info/src/convert_2q_block_matrix.rs	6	92.75%

Totals
Change from base Build 17500557721:	-0.07%
Covered Lines:	92787
Relevant Lines:	105073

---

💛 - Coveralls

[mtreinish]

This is a good start, thanks for looking at this in Rust this is a big improvement over doing it in Python. I have a few suggestions on how to make the code more efficient inline. Besides those suggestions I think the biggest issue is that we shouldn't be accessing the inner sparse observable object public, the public api of the sparse observable should give us the tools to get the data we need already, and if it doesn't we should expand the api with methods to get the data as appropriate.

[mtreinish]

Thanks for taking this over, I think this is pretty close, I just had a couple inline comments.

[Cryoris]

LGTM thanks for the updates!