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/utils.cpp

@@ -1,81 +0,0 @@
-
-#include "mlx/utils.h"
-#include <pybind11/pybind11.h>
-#include <pybind11/stl.h>
-#include <optional>
-
-namespace py = pybind11;
-using namespace py::literals;
-using namespace mlx::core;
-
-// Slightly different from the original, with python context on init we are not
-// in the context yet. Only create the inner context on enter then delete on
-// exit.
-class PyStreamContext {
- public:
-  PyStreamContext(StreamOrDevice s) : _inner(nullptr) {
-    if (std::holds_alternative<std::monostate>(s)) {
-      throw std::runtime_error(
-          "[StreamContext] Invalid argument, please specify a stream or device.");
-    }
-    _s = s;
-  }
-
-  void enter() {
-    _inner = new StreamContext(_s);
-  }
-
-  void exit() {
-    if (_inner != nullptr) {
-      delete _inner;
-      _inner = nullptr;
-    }
-  }
-
- private:
-  StreamOrDevice _s;
-  StreamContext* _inner;
-};
-
-void init_utils(py::module_& m) {
-  py::class_<PyStreamContext>(m, "StreamContext", R"pbdoc(
-        A context manager for setting the current device and stream.
-
-        See :func:`stream` for usage.
-
-        Args:
-            s: The stream or device to set as the default.
-  )pbdoc")
-      .def(py::init<StreamOrDevice>(), "s"_a)
-      .def("__enter__", [](PyStreamContext& scm) { scm.enter(); })
-      .def(
-          "__exit__",
-          [](PyStreamContext& scm,
-             const std::optional<py::type>& exc_type,
-             const std::optional<py::object>& exc_value,
-             const std::optional<py::object>& traceback) { scm.exit(); });
-  m.def(
-      "stream",
-      [](StreamOrDevice s) { return PyStreamContext(s); },
-      "s"_a,
-      R"pbdoc(
-        Create a context manager to set the default device and stream.
-
-        Args:
-            s: The :obj:`Stream` or :obj:`Device` to set as the default.
-
-        Returns:
-            A context manager that sets the default device and stream.
-
-        Example:
-
-        .. code-block::python
-
-          import mlx.core as mx
-
-          # Create a context manager for the default device and stream.
-          with mx.stream(mx.cpu):
-              # Operations here will use mx.cpu by default.
-              pass
-      )pbdoc");
-}