Shared events for synchronization + async eval (#998)

* more async eval

* fix rebase

* try correct async eval

* fix async

* more tests for async eval

* use shared events for synchronization

* comment + cleanup

* with autorelease pool

* fix no metal build

* fix compile

* fix patch

* don't eval if asyn evale'd

* don't use is_evaled

* comments

* more multi stream tests

* try and cleanup use of is_evaled

* use a status flag

python/src/transforms.cpp

@@ -595,14 +595,6 @@ class PyCheckpointedFun {
 };
 
 void init_transforms(nb::module_& m) {
-  nb::class_<std::shared_future<void>>(
-      m,
-      "Synchronizer",
-      R"pbdoc(
-      A synchronization object returned by :func:`async_eval`.
-      )pbdoc")
-      .def("wait", [](const std::shared_future<void>& f) { f.wait(); });
-
   m.def(
       "eval",
       [](const nb::args& args) {
@@ -629,19 +621,14 @@ void init_transforms(nb::module_& m) {
         std::vector<array> arrays = tree_flatten(args, false);
         {
           nb::gil_scoped_release nogil;
-          return async_eval(arrays);
+          async_eval(arrays);
         }
       },
       nb::arg(),
-      nb::sig("def async_eval(*args) -> Synchronizer"),
+      nb::sig("def async_eval(*args)"),
       R"pbdoc(
         Asynchronously evaluate an :class:`array` or tree of :class:`array`.
 
-        .. warning::
-
-          You must call ``wait`` on the returned synchronization object before
-          using any arrays that are asynchronously evaluated.
-
         .. note::
 
           This is an experimental API and may change in future versions.
@@ -652,8 +639,17 @@ void init_transforms(nb::module_& m) {
               :class:`list`, :class:`tuple` or :class:`dict`. Leaves which are not
               arrays are ignored.
 
-        Returns:
-            Synchronizer: A synchronization object.
+        Example:
+            >>> x = mx.array(1.0)
+            >>> y = mx.exp(x)
+            >>> mx.async_eval(y)
+            >>> print(y)
+            >>>
+            >>> y = mx.exp(x)
+            >>> mx.async_eval(y)
+            >>> z = y + 3
+            >>> mx.async_eval(z)
+            >>> print(z)
       )pbdoc");
   m.def(
       "jvp",