Gpu test for VectorFieldPosterior

[jorobledo]

Following PR #1527 and issue #1530 , improved the test for vector field methods.

• Added testing for training and/or sampling on device. Cross test for cpu-gpu are handled by setting two devices.
• Removed testing both on FMPE and NPSE since they are redundant (both share log_prob implementation).
• Fixed minor bug on time tensor in _loss function not being on the same device when moving to device the training.
• Added docstrings for GPU tests.

This one is small! Let me know if anything is missing or you think that it can be improved @StarostinV @janfb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.94%. Comparing base (8900ca0) to head (d46cfbe).
Report is 9 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1542       +/-   ##
===========================================
- Coverage   89.45%   78.94%   -10.52%     
===========================================
  Files         128      133        +5     
  Lines       10170    10370      +200     
===========================================
- Hits         9098     8187      -911     
- Misses       1072     2183     +1111     

Flag	Coverage Δ
unittests	78.94% <100.00%> (-10.52%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../inference/trainers/npse/vector_field_inference.py	95.48% <100.00%> (ø)

... and 37 files with indirect coverage changes

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[janfb]

Looks good, thanks @jorobledo !

added a couple of small suggestions / typo fixes.

[janfb]

thanks @jorobledo , looks perfect now! 🎉