Hilbert Schmidt templates

[rmoyard]

Context:
From this paper about compilation of quantum circuits with quantum computers, it appears that some templates are useful for PennyLane, Hilbert Schmidt test and local Hilbert Schmidt test.

Description of the Change:

Add two templates HilbertSchmidt and HilbertSchmidtLocal that can be used to implement the information quantities from the paper.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2364 (5933ccd) into master (f385f5b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2364   +/-   ##
=======================================
  Coverage   99.46%   99.47%           
=======================================
  Files         242      243    +1     
  Lines       19170    19247   +77     
=======================================
+ Hits        19068    19145   +77     
  Misses        102      102           

Impacted Files	Coverage Δ
pennylane/optimize/nesterov_momentum.py	100.00% <100.00%> (ø)
pennylane/templates/subroutines/__init__.py	100.00% <100.00%> (ø)
pennylane/templates/subroutines/hilbert_schmidt.py	100.00% <100.00%> (ø)

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[antalszava]

This is very thoroughly written, looks very nice! @rmoyard 🎉

Had a few formatting/refactoring/wording suggestions, though no real blockers so would be happy to approve on a second quick pass.

Noticed two additional things in general:

• The changelog entry is missing;
• It seems that 8 linting issues are being uncovered by Codefactor.

[antalszava]

@rmoyard great push with polishing 🚀 💯 Very near the finish.

A couple of final comments, some from before.

• Codefactor still seems to have some issues: https://github.com/PennyLaneAI/pennylane/runs/6028015270;
• LocalHilbertSchmidt seems to not have an image, would that be intentional?
• The new templates should also appear on this page: https://pennylane--2364.org.readthedocs.build/en/2364/introduction/templates.html#other-subroutines . That can be achieved by adding a card for the two templates in the doc/introduction/templates.rst file;
• Also, the docs check seems to be failing: https://github.com/PennyLaneAI/pennylane/runs/6028888508?check_suite_focus=true.

[antalszava]

Looks good 🎉 🥇