[ROCm] Add hip_kernel stub to no_rocm.cpp to fix undefined symbol

When MLX_BUILD_ROCM=OFF, the Python binding unconditionally references
mlx::core::fast::hip_kernel but no_rocm.cpp only stubbed
rocm::is_available(). Add a throwing stub matching the pattern used by
no_metal.cpp and no_cuda.cpp.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Geramy

mlx/backend/rocm/no_rocm.cpp

@@ -1,11 +1,31 @@
 // Copyright © 2025 Apple Inc.
 
 #include "mlx/backend/rocm/rocm.h"
+#include "mlx/fast.h"
 
-namespace mlx::core::rocm {
+namespace mlx::core {
+
+namespace rocm {
 
 bool is_available() {
   return false;
 }
 
-} // namespace mlx::core::rocm
+} // namespace rocm
+
+namespace fast {
+
+CustomKernelFunction hip_kernel(
+    const std::string&,
+    const std::vector<std::string>&,
+    const std::vector<std::string>&,
+    const std::string&,
+    const std::string&,
+    bool,
+    int) {
+  throw std::runtime_error("[hip_kernel] No ROCm back-end.");
+}
+
+} // namespace fast
+
+} // namespace mlx::core