feat(gpu_prover): multi-GPU support

[robik75]

What ❔

This PR contains changes needed to make it possible to use multiple GPUs for proving from the same process.

Is this a breaking change?

• [x] Yes - implicit host allocator creation was removed from ProverContext constructor and needs to be called explicitly
• No

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Code has been formatted.