Fix calculation of device index in multi-gpu setting

[LeStoe11]

Description

Fix a bug where the model would index out-of-bounds devices.

Fixes # (issue)
#837

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• [ x] I have not rewritten tests relating to key interfaces which would affect backward compatibility

[LeStoe11]

Should be disregarded in favour of #907