Move pauli word logic from qml.grouping.utils.is_commuting to qml.is_commuting

[timmysilv]

Context:
There are currently two different implementations of is_commuting(): one at the top-level for a variety of operators, and another in pennylane/grouping/utils.py specifically for Pauli words. These should be in the same place to avoid drift, give a simpler user interface and ease future development, so I'm putting them in the same place (qml.is_commuting)

Description of the Change:

• Move the Pauli word is_commuting logic to qml.is_commuting() (thereby removing qml.grouping.utils.is_commuting()), and try to check this before other checks
• If it deems one or both operators to not be Pauli words, check if either are Tensors
  • If they are, raise an exception
  • Else, carry on
• Explicitly remove support for Prod and Sum classes (as well as Tensors) since we do not yet have an implementation to support them

Benefits:

• Single method for users to check if two operators commute. Distinguishing logic for Pauli words vs. other operators is hidden away.
• Explicitly removes support for (non-Pauli word) Tensors, as well as composite operators

Possible Drawbacks:

• qml.is_commuting() is becoming large and messy (as hinted by our pylint disabled features 😄 ). Hopefully this gets some love in the future. The code is still organized and legible, just a warning.

Related GitHub Issues:
Fixes #3007

[github-actions]

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[codecov]

Codecov Report

Merging #3033 (02616ac) into master (29acf0d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3033   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files         271      271           
  Lines       23161    23164    +3     
=======================================
+ Hits        23084    23087    +3     
  Misses         77       77           

Impacted Files	Coverage Δ
pennylane/grouping/__init__.py	100.00% <ø> (ø)
pennylane/grouping/utils.py	100.00% <ø> (ø)
pennylane/ops/functions/is_commuting.py	100.00% <100.00%> (ø)

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[AmintorDusko]

Nice work @timmysilv!

[albi3ro]

Great to have this simplification!