Fix SymbolicHamiltonian.expectation for different nqubits

[stavros11]

Fixes #935. Specifically:

• nqubits attribute was added to SymbolicHamiltonian as suggested in Wrong results using SymbolicHamiltonian #935 (comment). Note that the effect of this is still equivalent to padding, for example SymbolicHamiltonian(Z(0) * Z(1), nqubits=4) is equivalent to SymbolicHamiltonian(Z(0) * Z(1) * I(2) * I(3)).
• A ValueError is raised if there is a mismatch between the number of qubits in the state and the Hamiltonian in expectation value calculations.
• A documentation example about expectation_from_samples was added (@AlejandroSopena let me know if that is okay).

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (05e2d3c) 100.00% compared to head (23ce101) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #951   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           49        49           
  Lines         6905      6906    +1     
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+ Hits          6905      6906    +1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/hamiltonians/hamiltonians.py	100.00% <100.00%> (ø)
src/qibo/hamiltonians/models.py	100.00% <100.00%> (ø)

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[AlejandroSopena]

Thanks @stavros11 for adding the example. The tests are not passing because expectation_from_samples() only works when the observable is diagonal in the z basis as you mentioned, and XXZ() is not. I have pending to generalize this function #852 and I will do it at some point in August.

[AlejandroSopena]

Thanks. Just a few more comments. When adding the three CNOTs, you have to replace c --> circuit To use expectation_from_samples() we have to add measurement gates because the expected value is calculated from frequencies.

[stavros11]

Thanks for the fixes @AlejandroSopena. I had them locally in my computer but forgot to push. I hope it is okay now.