Fix doc deployment by moving torch import

[Simone-Bordoni]

Remove torch import at the beginning of the pytorch backend to fix test in contructing the documentation. Also improved the conversion to pytorch dtype.

Checklist:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (8fd5657) to head (e249e85).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1281   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          73       73           
  Lines       10690    10694    +4     
=======================================
+ Hits        10674    10678    +4     
  Misses         16       16           

Flag	Coverage Δ
unittests	99.85% <100.00%> (+<0.01%)	⬆️

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