feat(concrete_npe): remove torus type generics

[mayeul-zama]

Description

Replace remove torus type generics by a function parameter log2_modulus in all relevant functions in concrete-npe.

• the generics torus type in only ever used to extract the number of bits, so we don't loose any useful information by only giving the log2_modulus
• we only ask for the information we need so it's less constraining for callers
• the generics are contaminating, which forces us to use generics too in the optimizer (log2_modulus -> generic type) is not possible (or very ugly: match log2_modulus {32 => f::(...), 64 => f::(...))
• for another client of concrete-npe using generics, it's possible to get the log2_modulus from the generic type (genric_type -> log2_modulus) is clean (f(..., T::BITS))
• we add a function parameter but we also remove a generic type, so we slightly reduce signature complexity
• with generics, monomorphisation is done (for each type we call generic a function with, the compiler creates a separate version) which may increase compile times and binary size
• theoretical future advantage: with the log2_modulus, we can call the functions for any number of bits, which may be useful for a FPGA implementation which doesn't use 16, 32, 64 or 128 bits

Checklist

(Use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• The PR description links to the related issue (to link an issue, use '#XXX'.)
• The tests on AWS have been launched and are successful (comment with @slab-ci cpu_test and/or @slab-ci gpu_test to trigger the tests)
• The draft release description has been updated
• Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification

[cla-bot]

Thank you for your pull request. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @mayeul-zama on file. In order for us to review and merge your code, please send an email to hello@zama.ai to get yourself added

[mayeul-zama]

@slab-ci cpu_test

[mayeul-zama]

@slab-ci cpu_test

[agnesLeroy]

Hi @mayeul-zama! Thank you for the contribution! 🙂 It looks good to me, I just have a small comment you'll see. 😉

[agnesLeroy]

Just a small thing: we could use a u32 or even a u8 for log2_modulus I guess, it's always going to be 32 or 64...

[IceTDrinker]

I think rust uses u32 for these like when calling e.g. 2u64.pow(/* u32 here */)