Add option collect_from_back to CollectMultiQBlocks

[alexanderivrii]

Summary

This PR adds a new option collect_from_back to the transpiler pass CollectMultiQBlocks, leading to an increased flexibility of the pass. As an example, on the following circuit

             ┌───┐
        q_0: ┤ H ├──■────■────■───────
             └───┘┌─┴─┐  │    │
        q_1: ─────┤ X ├──┼────┼───────
                  └───┘┌─┴─┐  │
        q_2: ──────────┤ X ├──┼───────
                       └───┘┌─┴─┐┌───┐
        q_3: ───────────────┤ X ├┤ H ├
                            └───┘└───┘

running the pass with max_block_size=3 and collect_from_back=False (default), collects the first three gates (the H-gate and the two following CX-gates) into the first block, and the two remaining gates into the second block, while running the pass with max_block_size=3 and collect_from_back=True (new), collects the last three gates into the second block, and the remaining two gates into the first block.

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 12668777162

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.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 340 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.933%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
qiskit/circuit/library/standard_gates/rz.py	3	95.71%
qiskit/circuit/library/standard_gates/rx.py	3	95.95%
qiskit/circuit/library/standard_gates/ry.py	3	95.83%
crates/qasm2/src/lex.rs	5	91.73%
qiskit/primitives/containers/observables_array.py	5	95.0%
crates/qasm2/src/parse.rs	6	97.62%
crates/accelerate/src/commutation_checker.rs	16	97.41%
crates/accelerate/src/unitary_synthesis.rs	23	93.18%
qiskit/transpiler/passes/synthesis/hls_plugins.py	58	83.38%

Totals
Change from base Build 12630226525:	-0.02%
Covered Lines:	79436
Relevant Lines:	89321

---

💛 - Coveralls

[jakelishman]

This looks like a straightforward new feature and implementation, thanks. One minor testing question only.

[jakelishman]

Presumably you mean self.assertEqual (and in the other place)? This is asserting non-emptiness of the pass_.property_set["block_list"], with a "message" field of expected_blocks.

[alexanderivrii]

Oops, of course! Fixed in 65a1aba.

[jakelishman]

Cool, thanks!