[Bugfix 1999] Correct attributes

[mariaschuld]

Context:

It was noted that the existing attributes miss some gates and, in one case, contain a wrong gate.

Adding the IsingZZ to is_diagonal_in_z_basis uncovered an issue: Since it does not have _eigvals defined, the eigenvalues are computed from the matrix using a numpy module, which means they are not differentiable in a backpropagation pipeline.

I think this is an issue with inferring the eigenvalues from a matrix in a non-diffable manner, which goes against the grain of PL. For now we fix it by warning of this issue.

Description of the Change:

Remove/add gates. Add _eigvals method to IsingZZ and explain the _eigvals requirement in the attributes docstring.

Benefits: Fixes #1999

[cvjjm]

Nice work!

[josh146]

@mariaschuld I may hold off on reviewing until the tests are passing if that is okay!

[mariaschuld]

@mariaschuld I may hold off on reviewing until the tests are passing if that is okay!

Sorry @josh146 I should not have tagged you before checking the tests, this 20 min wait is too much for my impatience :)

[codecov]

Codecov Report

Merging #2113 (ce84181) into master (dd50a7f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2113   +/-   ##
=======================================
  Coverage   99.19%   99.19%           
=======================================
  Files         228      228           
  Lines       17534    17558   +24     
=======================================
+ Hits        17393    17417   +24     
  Misses        141      141           

Impacted Files	Coverage Δ
pennylane/drawer/representation_resolver.py	99.43% <ø> (ø)
pennylane/ops/qubit/__init__.py	100.00% <ø> (ø)
pennylane/ops/qubit/attributes.py	100.00% <ø> (ø)
pennylane/devices/default_qubit.py	100.00% <100.00%> (ø)
pennylane/drawer/tape_mpl.py	100.00% <100.00%> (ø)
pennylane/math/is_independent.py	100.00% <100.00%> (ø)
pennylane/math/multi_dispatch.py	100.00% <100.00%> (ø)
pennylane/ops/qubit/non_parametric_ops.py	97.43% <100.00%> (+0.05%)	⬆️
pennylane/ops/qubit/parametric_ops.py	99.83% <100.00%> (+<0.01%)	⬆️
pennylane/templates/state_preparations/basis.py	100.00% <100.00%> (ø)

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[josh146]

Looks good to me, thanks @mariaschuld!