[bugfix] Probabilities do not sum to one with Torch

[PietropaoloFrisoni]

Context: When computing the expval of an operator using a quantum device with the torch interface and default_dtype set to torch.float32, the probabilities do not sum to one. This error does not occur if default_dtype is set to torch.float64.

Description of the Change: A renormalization of probabilities is introduced to overcome the issue. Such renormalization of probabilities occurs whenever the following two conditions are satisfied: 1) There is at least one probability that does not sum precisely to one, and 2) Such difference for all probabilities is between 0 and 1e-07.

Benefits: The error is not raised anymore.

Possible Drawbacks: The main drawback is that renormalization can occur in cases where it should not happen, although it is unlikely since the cutoff 1e-07 is expected to be reasonably small to prevent such cases (but large enough not to raise the error anymore).

Related GitHub Issues: #5444

[sc-59957]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (3b0c1b6) to head (d11f2ba).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5462      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         402      402              
  Lines       37540    37287     -253     
==========================================
- Hits        37420    37166     -254     
- Misses        120      121       +1     

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[albi3ro]

Test?

[albi3ro]

Looks great! Thanks for taking this on 🚀