redesign for faster cpu/gpu synch (#1869)

* redesign for faster cpu/gpu synch

* load + more async CPU

* use command encoder API and move more ops to use it

* make fence back-end generic + CPU only fence

* faster build

* fix async eval

* fixes + handle temporaries

* fix / improve cpu conv

* remove unused status, fix siblings

* fix extensions

* fix

* fix no cpu build

* format

* comments

* fix perf regression, remove unecessary abort

* fix events, task limit cpu

* fix waiting

* fix donation / temporaries in normalization

mlx/backend/cpu/encoder.h

@@ -0,0 +1,53 @@
+// Copyright © 2025 Apple Inc.
+
+#pragma once
+
+#include <unordered_map>
+
+#include "mlx/array.h"
+#include "mlx/scheduler.h"
+
+namespace mlx::core::cpu {
+
+struct CommandEncoder {
+  CommandEncoder(Stream stream) : stream_(stream) {}
+
+  CommandEncoder(const CommandEncoder&) = delete;
+  CommandEncoder& operator=(const CommandEncoder&) = delete;
+  CommandEncoder(CommandEncoder&&) = delete;
+  CommandEncoder& operator=(CommandEncoder&&) = delete;
+
+  void set_input_array(const array& a) {}
+  void set_output_array(array& a) {}
+
+  // Hold onto a temporary until any already scheduled tasks which use it as
+  // an input are complete.
+  void add_temporary(array arr) {
+    temporaries_.push_back(std::move(arr));
+  }
+
+  void add_temporaries(std::vector<array> arrays) {
+    temporaries_.insert(
+        temporaries_.end(),
+        std::make_move_iterator(arrays.begin()),
+        std::make_move_iterator(arrays.end()));
+  }
+
+  std::vector<array>& temporaries() {
+    return temporaries_;
+  }
+
+  template <class F, class... Args>
+  void dispatch(F&& f, Args&&... args) {
+    auto task = std::bind(std::forward<F>(f), std::forward<Args>(args)...);
+    scheduler::enqueue(stream_, std::move(task));
+  }
+
+ private:
+  Stream stream_;
+  std::vector<array> temporaries_;
+};
+
+CommandEncoder& get_command_encoder(Stream stream);
+
+} // namespace mlx::core::cpu