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

mlx/event.h

@@ -0,0 +1,56 @@
+// Copyright © 2024 Apple Inc.
+#pragma once
+
+#include <memory>
+#include <stdexcept>
+
+#include "mlx/stream.h"
+
+namespace mlx::core {
+
+class Event {
+ public:
+  Event(){};
+
+  Event(const Stream& steam);
+
+  // Wait for the event to be signaled at its curent value
+  void wait();
+
+  // Signal the event at its current value
+  void signal();
+
+  // Check if the event is valid
+  bool valid() {
+    return event_ != nullptr;
+  };
+
+  uint64_t value() {
+    return value_;
+  };
+
+  void set_value(uint64_t v) {
+    value_ = v;
+  };
+
+  const Stream& stream() {
+    if (!valid()) {
+      throw std::runtime_error(
+          "[Event::stream] Cannot access stream on invalid event.");
+    }
+    return stream_;
+  };
+
+  const std::shared_ptr<void>& raw_event() {
+    return event_;
+  };
+
+ private:
+  // Default constructed stream should never be used
+  // since the event is not yet valid
+  Stream stream_{0, Device::cpu};
+  std::shared_ptr<void> event_{nullptr};
+  uint64_t value_{0};
+};
+
+} // namespace mlx::core