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/eval.cpp

@@ -0,0 +1,44 @@
+// Copyright © 2025 Apple Inc.
+#include "mlx/backend/cpu/eval.h"
+#include "mlx/backend/cpu/encoder.h"
+#include "mlx/primitives.h"
+#include "mlx/scheduler.h"
+#include "mlx/utils.h"
+
+namespace mlx::core::cpu {
+
+void eval(array& arr) {
+  auto s = arr.primitive().stream();
+
+  auto outputs = arr.outputs();
+  {
+    // If the array is a tracer hold a reference
+    // to its inputs so they don't get donated
+    std::vector<array> inputs;
+    if (arr.is_tracer()) {
+      inputs = arr.inputs();
+    }
+    arr.primitive().eval_cpu(arr.inputs(), outputs);
+  }
+
+  std::unordered_set<std::shared_ptr<array::Data>> buffers;
+  for (auto& in : arr.inputs()) {
+    buffers.insert(in.data_shared_ptr());
+  }
+  for (auto& s : arr.siblings()) {
+    buffers.insert(s.data_shared_ptr());
+  }
+  // Remove the output if it was donated to by an input
+  if (auto it = buffers.find(arr.data_shared_ptr()); it != buffers.end()) {
+    buffers.erase(it);
+  }
+  auto& encoder = cpu::get_command_encoder(s);
+  scheduler::notify_new_task(s);
+  encoder.dispatch([s,
+                    buffers = std::move(buffers),
+                    temps = std::move(encoder.temporaries())]() {
+    scheduler::notify_task_completion(s);
+  });
+}
+
+} // namespace mlx::core::cpu