Fix nax condition for iphone (#3083)

Author: Awni Hannun

CMakeLists.txt

@@ -329,7 +329,7 @@ else()
   FetchContent_Declare(
     fmt
     GIT_REPOSITORY https://github.com/fmtlib/fmt.git
-    GIT_TAG 10.2.1
+    GIT_TAG 12.1.0
     EXCLUDE_FROM_ALL)
   FetchContent_MakeAvailable(fmt)
 endif()

mlx/backend/metal/device.h

@@ -275,8 +275,10 @@ inline bool is_nax_available() {
             macOS 26.2, iOS 26.2, tvOS 26.2, visionOS 26.2, *)) {
       can_use_nax = true;
     }
-    can_use_nax &=
-        metal::device(mlx::core::Device::gpu).get_architecture_gen() >= 17;
+    auto& d = metal::device(mlx::core::Device::gpu);
+    auto arch = d.get_architecture().back();
+    auto gen = d.get_architecture_gen();
+    can_use_nax &= gen >= (arch == 'p' ? 18 : 17);
     return can_use_nax;
   };
   static bool is_nax_available_ = _check_nax();