Fix parameter counting in DefaultQubitLegacy adjoint method

[dwierichs]

Context:
The adjoint method of DefaultQubitLegacy uses two incompatible ways to quantify tape parameters: It sets up the number of tape parameters correctly using tape.get_parameters(trainable_only=False) but checks whether an operation comes with a parameter via grad_method is not None. This lead to a bug where, for example, QubitUnitary disrupts the parameter counting.

Description of the Change:
As all operations with more than one parameter are being decomposed/raised an error upon anyways, this PR changes the way to detect parametrized operations to op.num_params==1.

Benefits:
Bug fix: Correct adjoint method derivatives even when using QubitUnitary and other operations that have grad_method=None but have parameters.

Possible Drawbacks:
N/A

Related GitHub Issues:
N/A

[dwierichs]

@vincentmr This is the PR (sorry, took a moment to figure this out after all)

[albi3ro]

Even though its currently not causing issues, mind fixing this as well in the new system:

pennylane/pennylane/devices/qubit/adjoint_jacobian.py

Line 83 in 52b9f5c

if op.grad_method is not None:

pennylane/pennylane/devices/qubit/adjoint_jacobian.py

Line 160 in 52b9f5c

if op.grad_method is not None:

pennylane/pennylane/devices/qubit/adjoint_jacobian.py

Line 235 in 52b9f5c

if op.grad_method is not None:

[dwierichs]

Even though its currently not causing issues, mind fixing this as well in the new system:

@albi3ro Done :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (677fbe0) 99.64% compared to head (5f65309) 99.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4820      +/-   ##
==========================================
- Coverage   99.64%   99.64%   -0.01%     
==========================================
  Files         381      381              
  Lines       34377    34117     -260     
==========================================
- Hits        34256    33995     -261     
- Misses        121      122       +1     

Files	Coverage Δ
pennylane/_qubit_device.py	99.44% <100.00%> (-0.01%)	⬇️
pennylane/devices/qubit/adjoint_jacobian.py	100.00% <100.00%> (ø)

... and 42 files with indirect coverage changes

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[timmysilv]

great find, and simple fix!

[albi3ro]

👍 Thanks for getting this together so quickly.