Add a call operator for CladFunction

Fixes #1654

Author: Vedant2005goyal

include/clad/Differentiator/Differentiator.h

@@ -444,6 +444,11 @@ CUDA_HOST_DEVICE void push(tape<T[N], SBO_SIZE, SLAB_SIZE>& to, const U& val) {
       return static_cast<return_type_t<F>>(0);
     }
 
+    template <typename... Args>
+    constexpr CUDA_HOST_DEVICE auto operator()(Args&&... args) const {
+      return execute(std::forward<Args>(args)...);
+    }
+
     /// Return the string representation for the generated derivative.
     constexpr const char* getCode() const {
       if (m_Code)

test/Features/DiffInterfaceExec.C

@@ -0,0 +1,31 @@
+// RUN: %cladclang -std=c++17 -I%S/../../include %s -o %t
+// RUN: %t | %filecheck_exec %s
+
+#include "clad/Differentiator/Differentiator.h"
+#include <cstdio>
+
+double f(double x) {
+  return x * x;
+}
+
+int main() {
+  auto f_diff = clad::differentiate(f);
+  
+  // Test the new operator() syntax
+  double res_diff = f_diff(3.0); 
+  printf("Diff result: %.2f\n", res_diff);
+  // CHECK-EXEC: Diff result: 6.00
+
+  // 2. Test gradient (Reverse Mode)
+  auto f_grad = clad::gradient(f);
+  double x = 4.0;
+  double d_x = 0.0;
+  
+  // Test the new operator() syntax
+  f_grad(x, &d_x);
+  
+  printf("Grad result: %.2f\n", d_x);
+  // CHECK-EXEC: Grad result: 8.00
+
+  return 0;
+}