Support pickling array for bfloat16 (#2586)

* add bfloat16 pickling

* Improvements

* improve

---------

Co-authored-by: Chen-Chen Yeh <ge96noj@mytum.de>

python/src/convert.h

@@ -12,6 +12,10 @@
 namespace mx = mlx::core;
 namespace nb = nanobind;
 
+namespace nanobind {
+static constexpr dlpack::dtype bfloat16{4, 16, 1};
+}; // namespace nanobind
+
 struct ArrayLike {
   ArrayLike(nb::object obj) : obj(obj) {};
   nb::object obj;