Switch to nanobind (#839)

* mostly builds

* most tests pass

* fix circle build

* add back buffer protocol

* includes

* fix for py38

* limit to cpu device

* include

* fix stubs

* move signatures for docs

* stubgen + docs fix

* doc for compiled function, comments

python/src/convert.h

@@ -0,0 +1,16 @@
+// Copyright © 2024 Apple Inc.
+
+#include <optional>
+
+#include <nanobind/nanobind.h>
+#include <nanobind/ndarray.h>
+
+#include "mlx/array.h"
+
+namespace nb = nanobind;
+using namespace mlx::core;
+
+array nd_array_to_mlx(
+    nb::ndarray<nb::ro, nb::c_contig, nb::device::cpu> nd_array,
+    std::optional<Dtype> dtype);
+nb::ndarray<nb::numpy> mlx_to_np_array(const array& a);