Prepare multi gpu support

[agnesLeroy]

closes: please link all relevant issues

PR content/description

This PR prepares the code base for multi-gpu support. At the moment all functions fall back to using only GPU 0, but we'll be able to add multi-GPU support little by little on top of this.

Check-list:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] Relevant issues are marked as resolved/closed, related issues are linked in the description
• [ ] Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification