chore(gpu): add tfhe-cuda-backend to the workspace

[agnesLeroy]

closes: https://github.com/zama-ai/tfhe-rs-internal/issues/442 and https://github.com/zama-ai/tfhe-rs-internal/issues/440

PR content/description

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

[github-actions]

@slab-ci gpu_test

[github-actions]

@slab-ci gpu_test

[IceTDrinker]

Thanks 🙂

[github-actions]

Pull Request has been approved 🎉
Launching full test suite...
@slab-ci cpu_test
@slab-ci cpu_unsigned_integer_test
@slab-ci cpu_signed_integer_test
@slab-ci cpu_wasm_test
@slab-ci csprng_randomness_testing