Add AAVQE example to doctests

[mlazzarin]

I added the AAVQE example to the doctests, fixed some typos, and removed some hardcoded calls to NumPy methods that make tests crash with CuPy.

[codecov]

Codecov Report

Merging #518 (195520b) into master (fe91300) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #518   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           83        83           
  Lines        11917     11917           
=========================================
  Hits         11917     11917           

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

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

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[scarrazza]

Looks good to me.

[mlazzarin]

@scarrazza what's your opinion about keeping the old main example main_old.py. Is it really necessary?

[scarrazza]

I think this file should be removed.