bugfix: BoxUniform device handling.

[janfb]

potential fix for #853.

a) We raise an error if the device passed via low/high does not match the device kward. E.g., if low/high have been moved to GPU already, but device is still on default "cpu".
b) Alternatively, we could automatically use the device passed via low/high as the new device to be less verbose and to follow the user's obvious intention to have the prior on that device.

For now, I implemented b). what do you think @michaeldeistler?

[codecov]

Codecov Report

Merging #854 (8bb676b) into main (86e73a9) will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main     #854      +/-   ##
==========================================
+ Coverage   74.59%   74.61%   +0.01%     
==========================================
  Files          80       80              
  Lines        6236     6244       +8     
==========================================
+ Hits         4652     4659       +7     
- Misses       1584     1585       +1     

Flag	Coverage Δ
unittests	74.61% <83.33%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
sbi/utils/torchutils.py	64.62% <83.33%> (+0.79%)	⬆️

... and 3 files with indirect coverage changes

[michaeldeistler]

I prefer option b)

Thanks a lot, LGTM!