Refactor common into cpu specific and truly common (#1817)

* refactor

* fix extension example

* fix no-cpu

mlx/backend/cpu/copy.h

@@ -0,0 +1,24 @@
+// Copyright © 2023-2024 Apple Inc.
+
+#pragma once
+
+#include "mlx/array.h"
+#include "mlx/backend/common/copy.h"
+#include "mlx/backend/common/utils.h"
+
+namespace mlx::core {
+
+void copy(const array& src, array& dst, CopyType ctype);
+void copy_inplace(const array& src, array& dst, CopyType ctype);
+
+void copy_inplace(
+    const array& src,
+    array& dst,
+    const Shape& data_shape,
+    const Strides& i_strides,
+    const Strides& o_strides,
+    int64_t i_offset,
+    int64_t o_offset,
+    CopyType ctype);
+
+} // namespace mlx::core