Generalize gpu backend (#2138)

* generalize gpu backend

* fix no_gpu build

* fix no_gpu build

* generalize gpu backend

mlx/backend/no_gpu/fence.cpp

@@ -0,0 +1,54 @@
+// Copyright © 2024 Apple Inc.
+
+#include <condition_variable>
+#include <mutex>
+
+#include "mlx/fence.h"
+#include "mlx/scheduler.h"
+
+namespace mlx::core {
+
+struct FenceImpl {
+  uint32_t count{0};
+  uint32_t value{0};
+  std::mutex mtx;
+  std::condition_variable cv;
+};
+
+Fence::Fence(Stream) {
+  auto dtor = [](void* ptr) { delete static_cast<FenceImpl*>(ptr); };
+  fence_ = std::shared_ptr<void>(new FenceImpl{}, dtor);
+}
+
+void Fence::wait(Stream stream, const array&) {
+  auto& f = *static_cast<FenceImpl*>(fence_.get());
+  if (stream.device == Device::cpu) {
+    scheduler::enqueue(stream, [count = f.count, fence_ = fence_]() mutable {
+      auto& f = *static_cast<FenceImpl*>(fence_.get());
+      std::unique_lock<std::mutex> lk(f.mtx);
+      if (f.value >= count) {
+        return;
+      }
+      f.cv.wait(lk, [&f, count] { return f.value >= count; });
+    });
+  } else {
+    throw std::runtime_error("[Fence::wait] Invalid stream.");
+  }
+}
+
+void Fence::update(Stream stream, const array&) {
+  auto& f = *static_cast<FenceImpl*>(fence_.get());
+  f.count++;
+  if (stream.device == Device::cpu) {
+    scheduler::enqueue(stream, [count = f.count, fence_ = fence_]() mutable {
+      auto& f = *static_cast<FenceImpl*>(fence_.get());
+      std::unique_lock<std::mutex> lk(f.mtx);
+      f.value = count;
+      f.cv.notify_all();
+    });
+  } else {
+    throw std::runtime_error("[Fence::update] Invalid stream.");
+  }
+}
+
+} // namespace mlx::core