Revert changes to DepolarizingChannel

[renatomello]

Somehow #890 reverted the changes implemented by #889 and we didn't notice. This PR reimplements those changes.

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 (9fad0b6) 100.00% compared to head (1f46cd6) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #903   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         6593      6573   -20     
=========================================
- Hits          6593      6573   -20     

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/gates/channels.py	100.00% <100.00%> (ø)
src/qibo/models/utils.py	100.00% <100.00%> (ø)
src/qibo/models/variational.py	100.00% <100.00%> (ø)

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