[WIP] Avoid duplicate diagonalizing_gates

[lillian542]

Context:
Currently, diagonalizing_gates for a tape just iterates over all the observables and adds their diagonalizing gates to the list. This means that if the same observable occurs more than once, it's diagonalizing gates are included more than once.

>>> tape = qml.tape.QuantumScript([], [qml.expval(X(0)), qml.var(X(0))])
>>> tape.diagonalizing_gates
[Hadamard(wires=[0]), Hadamard(wires=[0])]

Description of the Change:
We get diagonalizing gates from a set of the base observables instead of self.observables.

Benefits:

>>> tape = qml.tape.QuantumScript([], [qml.expval(X(0)), qml.var(X(0))])
>>> tape.diagonalizing_gates
[Hadamard(wires=[0])]

---

Open question:
One slightly unintuitive thing here is how to handle cases with CompositeOp observable where an observable is duplicated, but in an observable that can't be divided because its terms don't commute. For example, currently in this PR for:

tape.observables = [qml.X(0), qml.X(0)+qml.Y(0)]

Here we include X(0).diagonalizing_gates() and the QubitUnitary that diagonalized qml.X(0)+qml.Y(0). Since the sum can't be diagonalized as a single observable we treat is as a completely different observable, the way we would:

tape.observables = [qml.X(0), qml.Y(0)]

In contrast, if the sum doesn't have overlapping wires, and thus would be diagonalized one term at a time instead of as a single observable:

tape.observables = [qml.X(0), qml.X(0)+qml.Y(1)]

we end up with X(0).diagonalizing_gates() + Y(1).diagonalizing_gates()

This feels more consistent now that we have a diagonalizing gate for X(0)+Y(0), but it doesn't reflect how we currently diagonalize (currently we would split the terms on to separate tapes and use X(0).diagonalizing_gates() on one and Y(0).diagonalizing_gates() on the other. So maybe we should remove the special treatment for CompositeOps with overlapping wires and just let the diagonalizing gates for [qml.X(0), qml.X(0)+qml.Y(0)] be X(0).diagonalizing_gates() + Y(0).diagonalizing_gates().

[sc-64695]

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (4ae7113) to head (db62305).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6288      +/-   ##
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+ Coverage   99.69%   99.70%   +0.01%     
==========================================
  Files         444      444              
  Lines       42133    42163      +30     
==========================================
+ Hits        42004    42040      +36     
+ Misses        129      123       -6     

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