Rename exec_aten:: to executorch::aten:: (#5296)

Summary:
Pull Request resolved: #5296

Modify any files that declare a `namespace exec_aten` to use the new `executorch::aten` namespace, and add an alias for the old namespace. Migration of users will happen in later diffs.

Reviewed By: mergennachin

Differential Revision: D62528437

fbshipit-source-id: 2ca7db5904dbc88cd97623abe0c0d67647fd0e5b

Author: dbort

runtime/core/exec_aten/exec_aten.h

@@ -46,9 +46,10 @@
 
 #endif
 
-namespace exec_aten {
+namespace executorch {
+namespace aten {
 
-using TensorShapeDynamism = torch::executor::TensorShapeDynamism;
+using TensorShapeDynamism = executorch::runtime::TensorShapeDynamism;
 
 #ifdef USE_ATEN_LIB
 
@@ -134,13 +135,17 @@ using OptionalArrayRef =
 
 using torch::executor::compute_numel;
 
-#endif // Use executor types
+#endif // Use ExecuTorch types
 
-} // namespace exec_aten
+} // namespace aten
+} // namespace executorch
+
+// DEPRECATED: The exec_aten:: namespace is deprecated. Use executorch::aten::
+// instead.
+namespace exec_aten = executorch::aten;
 
 namespace torch {
 namespace executor {
 using TensorList = exec_aten::TensorList;
-
 } // namespace executor
 } // namespace torch