Update logic for enabling grad_on_execution in lightning_*.py devices

[andrijapau]

Context:

In PennyLane, grad_on_execution is expected to be enabled when the gradient method is set to adjoint. Since this is automatically true for Lightning devices, the use_device_gradient flag can serve as an additional condition to decide whether to enable grad_on_execution.

This discrepancy caused certain Lightning device tests to fail during a PennyLane workflow clean-up PR. The issue arose because PennyLane expected grad_on_execution to be None but received True instead. Further details can be found in the PR workflow.

Description of the Change:

Update logic for when grad_on_execution is turned on in lightning device configurations.

Benefits: Unblocks PennyLaneAI/pennylane#6657

Possible Drawbacks: None.
[sc-79648]
[sc-72150]

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 27.86%. Comparing base (158d393) to head (764829f).
Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
pennylane_lightning/lightning_gpu/lightning_gpu.py	0.00%	1 Missing ⚠️
...ane_lightning/lightning_kokkos/lightning_kokkos.py	0.00%	1 Missing ⚠️
...ylane_lightning/lightning_qubit/lightning_qubit.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (158d393) and HEAD (764829f). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (158d393)	HEAD (764829f)
	8	6

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1016       +/-   ##
===========================================
- Coverage   95.85%   27.86%   -68.00%     
===========================================
  Files         229       29      -200     
  Lines       38841     2584    -36257     
===========================================
- Hits        37232      720    -36512     
- Misses       1609     1864      +255     

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[maliasadi]

Thanks @andrijapau, LGTM!