Fix draw_detection to work with batch_size > 1

[sfmig]

Fix draw_detection to work with batch_size > 1

• atm draw_detection saves one image per batch, rather than every image.
• this PR fixes that by returning a list instead.

TODO

• review use of device in the visualisation function
  • in draw_detection we assign to the selected device, but then copy it back to the cpu.
• add tests to be more comprehensive
• refactor visualisation module