introduce QubitDevice._get_diagonalizing_gates; skip Hamiltonian in default.qubit

[timmysilv]

Context:
Can be done instead of #3929. Some devices compute diagonalizing gates, and this can be costly. If they aren't going to be used, a device should be able to skip trying.

Description of the Change:

1. Add _get_diagonalizing_gates to the QubitDevice class. It is a copy-paste of the QuantumScript.diagonalizing_gates property.
2. Replace circuit.diagonalizing_gates with self._get_diagonalizing_gates(circuit) in QubitDevice.execute (when it calls self.apply).
3. Override the method in DefaultQubit to filter out any measurements with isinstance(m.obs, qml.Hamiltonian).

Benefits:

• The device can weigh in on what rotation gates should be executed/evaluated for a circuit
• We move away from QuantumTape.diagonalizing_gates, whose reason for existence is questionable

Possible Drawbacks:

• More code in default.qubit that needs maintenance/porting over
• Duplicate of QuantumScript.diagonalizing_gates code

Additional Notes

• I put this in QubitDevice rather than Device to minimize changes to the outward-facing Device API, because I know the scope is limited. I can move it to Device if people prefer it, but this was an intentional choice
• I prefixed the method name with get_ on purpose, to show that it's doing something (unlike the aforementioned property which was intentionally hiding that fact), and to distinguish it from existing code. Again, I can change this if people prefer it

Related GitHub Issues:
#3929, blocks PennyLaneAI/pennylane-lightning#424

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #3938 (59bb0ef) into master (c957db9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3938   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files         345      345           
  Lines       30164    30171    +7     
=======================================
+ Hits        30086    30093    +7     
  Misses         78       78           

Impacted Files	Coverage Δ
pennylane/_qubit_device.py	99.38% <100.00%> (+<0.01%)	⬆️
pennylane/devices/default_qubit.py	99.70% <100.00%> (+<0.01%)	⬆️
pennylane/devices/null_qubit.py	100.00% <100.00%> (ø)
pennylane/tape/qscript.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[mudit2812]

I'm quite happy with these changes. I like the shift of moving diagonalizing gates to the device from the qscript a lot. Thanks Matt!

[albi3ro]

Make sure to add a changelog!

[albi3ro]

Looks good to me 👍