Fix some examples in docstrings

[mlazzarin]

In this PR I fixed some broken examples in the docstrings. I haven't made an exhaustive search so some examples may still be broken.

Let me know if it is all ok, in particular with the VariationalLayer (I had to change a lot to fix it)

[codecov]

Codecov Report

Merging #480 (328d487) into docsupdates (a5e6ecb) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           docsupdates      #480   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files               85        85           
  Lines            12084     12084           
=============================================
  Hits             12084     12084           

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

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Impacted Files	Coverage Δ
src/qibo/abstractions/callbacks.py	100.00% <ø> (ø)
src/qibo/abstractions/gates.py	100.00% <ø> (ø)
src/qibo/models/evolution.py	100.00% <ø> (ø)
src/qibo/models/grover.py	100.00% <ø> (ø)
src/qibo/models/variational.py	100.00% <ø> (ø)

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[scarrazza]

Thanks @mlazzarin looks good to me.