feat(cuda): introduce cuda acceleration for the pbs and keyswitch

- a new crate concrete-cuda is added to the repository, containing some
Cuda implementations for the bootstrap and keyswitch and a Rust wrapping
to call them
- a new backend_cuda is added to concrete-core, with dedicated entities
whose memory is located on the GPU and engines that call the Cuda
accelerated functions

src/complex/operations.cuh

@@ -0,0 +1,87 @@
+#ifndef GPU_BOOTSTRAP_COMMON_CUH
+#define GPU_BOOTSTRAP_COMMON_CUH
+
+#include <cassert>
+#include <cstdint>
+#include <cstdio>
+
+#define SNT 1
+#define dPI 6.283185307179586231995926937088
+
+using sTorus = int32_t;
+// using Torus = uint32_t;
+using sTorus = int32_t;
+using u32 = uint32_t;
+using i32 = int32_t;
+
+//--------------------------------------------------
+// Basic double2 operations
+
+__device__ inline double2 conjugate(const double2 num) {
+  double2 res;
+  res.x = num.x;
+  res.y = -num.y;
+  return res;
+}
+
+__device__ inline void operator+=(double2 &lh, const double2 rh) {
+  lh.x += rh.x;
+  lh.y += rh.y;
+}
+
+__device__ inline void operator-=(double2 &lh, const double2 rh) {
+  lh.x -= rh.x;
+  lh.y -= rh.y;
+}
+
+__device__ inline double2 operator+(const double2 a, const double2 b) {
+  double2 res;
+  res.x = a.x + b.x;
+  res.y = a.y + b.y;
+  return res;
+}
+
+__device__ inline double2 operator-(const double2 a, const double2 b) {
+  double2 res;
+  res.x = a.x - b.x;
+  res.y = a.y - b.y;
+  return res;
+}
+
+__device__ inline double2 operator*(const double2 a, const double2 b) {
+  double xx = a.x * b.x;
+  double xy = a.x * b.y;
+  double yx = a.y * b.x;
+  double yy = a.y * b.y;
+
+  double2 res;
+  // asm volatile("fma.rn.f64 %0, %1, %2, %3;": "=d"(res.x) : "d"(a.x),
+  // "d"(b.x), "d"(yy));
+  res.x = xx - yy;
+  res.y = xy + yx;
+  return res;
+}
+
+__device__ inline double2 operator*(const double2 a, double b) {
+  double2 res;
+  res.x = a.x * b;
+  res.y = a.y * b;
+  return res;
+}
+
+__device__ inline void operator*=(double2 &a, const double2 b) {
+  double tmp = a.x;
+  a.x *= b.x;
+  a.x -= a.y * b.y;
+  a.y *= b.x;
+  a.y += b.y * tmp;
+}
+
+__device__ inline double2 operator*(double a, double2 b) {
+  double2 res;
+  res.x = b.x * a;
+  res.y = b.y * a;
+  return res;
+}
+
+#endif