Porting synth_qft_line to Rust

[Barath-T]

Summary

This PR ports synth_qft_line in qiskit/synthesis/qft/qft_decompose_lnn.py to Rust (Closes #12245).

Details and comments

The functionality remains the same and no new optimisations or logic is introduced.

Comment needed: PR #13929 introduces SynthesisData, which would allow for a cleaner refactoring.

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[Cryoris]

This looks very good! I only have two small comments below 🙂

[coveralls]

Pull Request Test Coverage Report for Build 14700264921

Details

• 71 of 71 (100.0%) changed or added relevant lines in 6 files are covered.
• 20 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 87.914%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	92.23%
qiskit/synthesis/permutation/permutation_reverse_lnn.py	15	31.82%

Totals
Change from base Build 14693758171:	-0.01%
Covered Lines:	74393
Relevant Lines:	84620

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💛 - Coveralls

[alexanderivrii]

Thanks for the contribution @Barath-T! I think this is in a good shape, modulo needing to adjust the Rust-level docstrings a bit.

Fyi, I have reworked #13929 to work with CircuitData directly (and in particular added convenience functions to append h, cp, cx gates), but for your PR keeping it in terms of LnnGatesVec is definitely better.

[Barath-T]

@alexanderivrii and @Cryoris Thank you for the review. I have updated the PR with the suggested changes. Please let me know if further refinements are needed. I'd also appreciate it if you can point me to other issues that i can contribute. Thank you in advance! :)

[alexanderivrii]

@Barath-T, sorry, my suggestion in the previous review was incorrect:
since

#[pyfunction]
#[pyo3(signature=(num_qubits, do_swaps=true, approximation_degree=0))]
pub fn synth_qft_line(

is treated as a Python function, it actually needs sphinx style documentation, exactly like you had previously (thanks to @Cryoris for explaining that to me). Could you please revert this change (but keep the one to append_h)? In some sense, this does not even matter, as the main documentation includes the one from synthesis/qft/qft_decompose_lnn.py and not the one from qft_decompose_lnn.rs, but it might be good to keep it correct just in case.

[alexanderivrii]

Thanks for the changes! This looks good on my side, @Cryoris do you have any additional comments?

[ShellyGarion]

LGTM. @Cryoris - do you have any further comments?