Remove "using namespace mlx::core" in benchmarks/examples (#1685)

* Remove "using namespace mlx::core" in benchmarks/examples

* Fix building example extension

* A missing one in comment

* Fix building on M chips

benchmarks/cpp/autograd.cpp

@@ -5,35 +5,35 @@
 #include "mlx/mlx.h"
 #include "time_utils.h"
 
-using namespace mlx::core;
+namespace mx = mlx::core;
 
 void time_value_and_grad() {
-  auto x = ones({200, 1000});
-  eval(x);
-  auto fn = [](array x) {
+  auto x = mx::ones({200, 1000});
+  mx::eval(x);
+  auto fn = [](mx::array x) {
     for (int i = 0; i < 20; ++i) {
-      x = log(exp(x));
+      x = mx::log(mx::exp(x));
     }
-    return sum(x);
+    return mx::sum(x);
   };
 
-  auto grad_fn = grad(fn);
+  auto grad_fn = mx::grad(fn);
   auto independent_value_and_grad = [&]() {
     auto value = fn(x);
     auto dfdx = grad_fn(x);
-    return std::vector<array>{value, dfdx};
+    return std::vector<mx::array>{value, dfdx};
   };
   TIME(independent_value_and_grad);
 
-  auto value_and_grad_fn = value_and_grad(fn);
+  auto value_and_grad_fn = mx::value_and_grad(fn);
   auto combined_value_and_grad = [&]() {
     auto [value, dfdx] = value_and_grad_fn(x);
-    return std::vector<array>{value, dfdx};
+    return std::vector<mx::array>{value, dfdx};
   };
   TIME(combined_value_and_grad);
 }
 
 int main() {
-  std::cout << "Benchmarks for " << default_device() << std::endl;
+  std::cout << "Benchmarks for " << mx::default_device() << std::endl;
   time_value_and_grad();
 }