Split cuDNN helpers into a separate header (#2491)

* Add RAII managed CudaGraph class

* Implement forward rms_norm with cuDNN

* Revert back to old rms norm kernel

mlx/backend/cuda/CMakeLists.txt

@@ -17,6 +17,7 @@ target_sources(
           ${CMAKE_CURRENT_SOURCE_DIR}/copy/copy_general_input.cu
           ${CMAKE_CURRENT_SOURCE_DIR}/conv.cpp
           ${CMAKE_CURRENT_SOURCE_DIR}/cuda.cpp
+          ${CMAKE_CURRENT_SOURCE_DIR}/cudnn_utils.cpp
           ${CMAKE_CURRENT_SOURCE_DIR}/device.cpp
           ${CMAKE_CURRENT_SOURCE_DIR}/eval.cpp
           ${CMAKE_CURRENT_SOURCE_DIR}/event.cu