Sample minimal subset of wires on LightningQubit

[vincentmr]

Before submitting

Please complete the following checklist when submitting a PR:

• All new features must include a unit test.
  If you've fixed a bug or added code that should be tested, add a test to the
  tests directory!

• All new functions and code must be clearly commented and documented.
  If you do make documentation changes, make sure that the docs build and
  render correctly by running make docs.

• Ensure that the test suite passes, by running make test.

• Add a new entry to the .github/CHANGELOG.md file, summarizing the
  change, and including a link back to the PR.

• Ensure that code is properly formatted by running make format.

When all the above are checked, delete everything above the dashed
line and fill in the pull request template.

---

Context:
With enough qubits and shots, it is wasteful to generate samples on all wires.

Description of the Change:
LightningQubit calls generate_samples(wires) on a minimal subset of wires when executing in finite-shot mode.

Benefits:
Faster sampling and execution.
We perform a simple benchmark of

@qml.qnode(dev)
def f():
    return qml.sample(wires)

where wires takes values [0], range(0, nq//2) and range(0, nq). For a large enough number of qubits and/or samples, we observe good speed-ups (up to 8x) for the 1-wire and half-wire cases. As expected, the all-wire case yields ~1x for all simulations.

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.78%. Comparing base (d1baa8f) to head (f53c78e).
Report is 89 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (d1baa8f) and HEAD (f53c78e). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (d1baa8f)	HEAD (f53c78e)
	12	5

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #813       +/-   ##
===========================================
- Coverage   98.77%   58.78%   -40.00%     
===========================================
  Files         114       17       -97     
  Lines       17858     1895    -15963     
===========================================
- Hits        17640     1114    -16526     
- Misses        218      781      +563     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[AmintorDusko]

The performance is excellent. Is there an actual coverage issue, or is codecov just being a nuisance?

[vincentmr]

The performance is excellent. Is there an actual coverage issue, or is codecov just being a nuisance?

The patch cov is 100%, but project cov is low because C++ tests didn't run.

[LuisAlfredoNu]

Nice job. The improvement in the performance is good

[AmintorDusko]

Thank you, @vincentmr! Super cool!

[vincentmr]

[sc-65127]