Update 4-term parameter shift coefficients

[josh146]

Description of the Change: Updates the four-term parameter-shift rule to use coefficients that minimize the variance as per https://arxiv.org/abs/2104.05695

Benefits: More optimal choice of coefficients

Possible Drawbacks: n/a

Related GitHub Issues: n/a

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
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[codecov]

Codecov Report

Merging #1206 (6530972) into master (e17dc37) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1206   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         145      145           
  Lines       10985    10985           
=======================================
  Hits        10779    10779           
  Misses        206      206           

Impacted Files	Coverage Δ
pennylane/ops/qubit.py	98.29% <100.00%> (ø)

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[antalszava]

Looks good to me! 💯 🎊 None of the suggestions are blockers.

[dwierichs]

This looks good to me, up to a tiny documentation issue: The latest change of the sign in the CRX docstring would need to be applied to CRY, CRZ and CRot as well, wouldn't it?

[josh146]

Thanks @dwierichs, have just made them all consistent :)

[dwierichs]

Great, thanks for carrying out this PR! :)