[CUDA] Implement Scan kernel (#2347)

* Contiguous scan

* Strided scan

* Enable tests

* Fix failing logaddexp test

* Use cexpf in Metal

mlx/backend/cuda/device/unary_ops.cuh

@@ -2,6 +2,8 @@
 
 #pragma once
 
+#include "mlx/backend/cuda/device/cexpf.cuh"
+#include "mlx/backend/cuda/device/cucomplex_math.cuh"
 #include "mlx/backend/cuda/device/fp16_math.cuh"
 #include "mlx/backend/cuda/device/utils.cuh"
 
@@ -150,8 +152,7 @@ struct Exp {
   template <typename T>
   __device__ T operator()(T x) {
     if constexpr (cuda::std::is_same_v<T, cuComplex>) {
-      auto m = exp(cuCrealf(x));
-      return {m * cos(cuCimagf(x)), m * sinh(cuCimagf(x))};
+      return detail::cexpf(x);
     } else {
       return exp(x);
     }
@@ -228,8 +229,25 @@ struct Log10 {
 
 struct Log1p {
   template <typename T>
-  __device__ T operator()(T x) {
-    return log1p(x);
+  __device__ T operator()(T z) {
+    if constexpr (cuda::std::is_same_v<T, cuComplex>) {
+      float x = cuCrealf(z);
+      float y = cuCimagf(z);
+      float zabs = cuCrealf(Abs{}(z));
+      float theta = atan2f(y, x + 1);
+      if (zabs < 0.5f) {
+        float r = x * (2 + x) + y * y;
+        if (r == 0) { // handle underflow
+          return {x, theta};
+        }
+        return {0.5f * log1pf(r), theta};
+      } else {
+        float z0 = hypotf(x + 1, y);
+        return {logf(z0), theta};
+      }
+    } else {
+      return log1p(z);
+    }
   }
 };
 
@@ -387,19 +405,19 @@ struct Tanh {
   }
 };
 
-__device__ cuComplex ArcCos::operator()(cuComplex x) {
+inline __device__ cuComplex ArcCos::operator()(cuComplex x) {
   auto i = cuComplex{0.0, 1.0};
   auto y = Log{}(x + i * Sqrt{}(1.0 - x * x));
   return {cuCimagf(y), -cuCrealf(y)};
 };
 
-__device__ cuComplex ArcSin::operator()(cuComplex x) {
+inline __device__ cuComplex ArcSin::operator()(cuComplex x) {
   auto i = cuComplex{0.0f, 1.0f};
   auto y = Log{}(i * x + Sqrt{}(1.0f - x * x));
   return {cuCimagf(y), -cuCrealf(y)};
 };
 
-__device__ cuComplex ArcTan::operator()(cuComplex x) {
+inline __device__ cuComplex ArcTan::operator()(cuComplex x) {
   auto i = cuComplex{0.0f, 1.0f};
   auto ix = i * x;
   return (1.0f / cuComplex{0.0f, 2.0f}) * Log{}((1.0f + ix) / (1.0f - ix));