CUDA Scalar Mul

[jon-chuang]

Exposes single function cpu_gpu_scalar_mul

impl<G: AffineCurve> GPUScalarMulSlice<G> for [G] {
    fn cpu_gpu_scalar_mul(
        &mut self,
        exps_h: &[<<G as AffineCurve>::ScalarField as PrimeField>::BigInt],
        cuda_group_size: usize,
        // size of the batch for cpu scalar mul
        cpu_chunk_size: usize,
    ) {
        if accel::Device::init() && cfg!(feature = "cuda") {
            <G as AffineCurve>::Projective::cpu_gpu_static_partition_run_kernel(
                self,
                exps_h,
                cuda_group_size,
                cpu_chunk_size,
            );
        } else {
            let mut exps_mut = exps_h.to_vec();
            cfg_chunks_mut!(self, cpu_chunk_size)
                .zip(cfg_chunks_mut!(exps_mut, cpu_chunk_size))
                .for_each(|(b, s)| {
                    b[..].batch_scalar_mul_in_place(&mut s[..], 4);
                });
        }
    }
}

---

A majority of the PR lies in the algebra/curves/cuda folder (850 out of 1400 loc). There is a deletion of sw_projective.rs as it is not actually used anywhere and Pratyush mentioned he plans to drop it in future zexe. The rest of the diff is boilerplate to impl for different curves, or code displacement (300 loc). Also, there is a test.

Potential TODOs:

• cleanup the GPUScalarMul interface, choose what to expose (ctx?). Maybe, make GPUScalarMul pub(crate) instead of pub, and have GPUScalarMulSlice be the only pub.
• Alternatively, make a new trait, GPUScalarMulInternal pub(crate) and choose what to expose in public GPUScalarMul interface. (I prefer this).