Custom primitive + RoPE fat op (#676)

* extensions start

* rope custom op

* fix build

* docs + rope benchmark

* fix test

* Add a Metal kernel for RoPE

* Fix position of traditional

* transform tests

* Move rope computation to float and fix tests

* Fix the test and a typo

* change to fast

* fix no metal build

---------

Co-authored-by: Angelos Katharopoulos <a_katharopoulos@apple.com>

mlx/backend/metal/kernels/CMakeLists.txt

@@ -23,6 +23,7 @@ set(
   "quantized"
   "random"
   "reduce"
+  "rope"
   "scan"
   "softmax"
   "sort"