CNOT decomposition method returns DecompositionUndefinedError

[lillian542]

Context:
Before CNOT inherited from Controlled, its decomposition method returned a DecompositionUndefinedError. Now it uses the smart decomposition functions from Controlled, which in this case make it decompose to itself.

>>> op = qml.CNOT([0, 1])
>>> op.decomposition()
[CNOT(wires=[0, 1])]

The change in the decomposition method of CNOT was unintentional, and operators should not decompose to themselves.

Description of the Change:
We put it back how it was:

>>> op = qml.CNOT([0, 1])
>>> op.decomposition()
DecompositionUndefinedError

Related GitHub Issues:
#5711

[lillian542]

[sc-63619]

[dwierichs]

Looks good to me overall, leaving a preliminary review :)
It might be good to add an explicit test of the CNOT decomposition, in case you were not planning to do that anyways. :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (b7675ca) to head (0744db6).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6039      +/-   ##
==========================================
- Coverage   99.66%   99.65%   -0.01%     
==========================================
  Files         428      430       +2     
  Lines       41182    41169      -13     
==========================================
- Hits        41042    41028      -14     
- Misses        140      141       +1     

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[dwierichs]

Thanks @lillian542
Would be nice to update the comment regarding mapped_op.has_decomposition, otherwise LGTM! 🚀

[albi3ro]

👍 minor, non-blocking suggestions.