chore(gpu): parallelize keyswitch further

closes: please link all relevant issues

[ ] 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

zama-ai / tfhe-rs