Fix set_device test for GPU

[stavros11]

Fixes #415. It seems that the issue comes from a set_device test that does not reset the device properly back to GPU when it runs for the numpy backend. All tests now work both on CPU and GPU for me. @scarrazza please confirm that it is okay for you too.

[scarrazza]

Thank you, looks good to me and works fine on my systems.

[codecov]

Codecov Report

Merging #417 (be18e6a) into master (3b61c09) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #417   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           79        79           
  Lines        11399     11401    +2     
=========================================
+ Hits         11399     11401    +2     

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

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Impacted Files	Coverage Δ
src/qibo/tests/test_parallel.py	100.00% <ø> (ø)
src/qibo/parallel.py	100.00% <100.00%> (ø)
src/qibo/tests/test_backends_init.py	100.00% <100.00%> (ø)
src/qibo/tests/test_models_variational.py	100.00% <100.00%> (ø)

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