qml.QDrift template

[KetpuntoG]

Note: I have left in comments some doubts

Context:

Third of 3 PRs adding the new QDrift template to allow for advanced Trotter methods in Pennylane

Description of the Change:

Implement the template

Quick test to show that the solution is correct

It has been checked if at infinity (n = 10000) it converges to the exact value.

coeffs = [-1.1, .5]
ops = [qml.PauliX(0)@qml.PauliY(1), qml.PauliZ(1)]

H = qml.dot(coeffs, ops)
H1 = qml.Hamiltonian(coeffs, ops)

n = 10000
time = 0.1

qdrift_matrix1 = qml.matrix(qml.QDrift(H, time, n))
qdrift_matrix2 = qml.matrix(qml.QDrift(H1, time, n))
trotter_matrix = qml.matrix(qml.exp(H1, 1j * time))

print(np.round(qdrift_matrix1, 2))
print(np.round(qdrift_matrix2, 2))
print(np.round(trotter_matrix, 2))

Running it shows that the three matrices are equal

[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.
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[KetpuntoG]

Questions and Reminders

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8166719) 99.64% compared to head (559ee72) 99.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4671      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.01%     
==========================================
  Files         377      378       +1     
  Lines       34015    33810     -205     
==========================================
- Hits        33894    33688     -206     
- Misses        121      122       +1     

Files	Coverage Δ
pennylane/templates/subroutines/__init__.py	100.00% <100.00%> (ø)
pennylane/templates/subroutines/qdrift.py	100.00% <100.00%> (ø)

... and 42 files with indirect coverage changes

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[Jaybsoni]

[sc-43477]

[KetpuntoG]

Great job! 😊
And thanks for adding the error, I forgot it in the draft

[Jaybsoni]

Wow what a great PR, who ever coded this must be a really good developer 💯!

[soranjh]

Thanks @KetpuntoG and @Jaybsoni. Will approve when my final small comments are addressed.