Fix API warning for older SDKs #2 (pytorch#5365)

Summary:
Fix remaining warnings when building MPS delegate with older iOS SDKs

cc shoumikhin, cccclai

Pull Request resolved: pytorch#5365

Reviewed By: kirklandsign

Differential Revision: D62668033

Pulled By: shoumikhin

fbshipit-source-id: efcb8449b1f38259914c76337e12dc38487bf36b

backends/apple/mps/runtime/operations/OperationUtils.mm

@@ -31,8 +31,13 @@
       return MPSDataTypeFloat32;
     case DataType::mps_data_type_int8:
       return MPSDataTypeInt8;
-    case DataType::mps_data_type_int4:
-      return MPSDataTypeInt4;
+    case DataType::mps_data_type_int4: {
+      if (@available(macOS 15.0, iOS 18.0, tvOS 18.0, *)) {
+        return MPSDataTypeInt4;
+      } else {
+        return ((MPSDataType)(MPSDataTypeSignedBit | 4));
+      }
+    }
     case DataType::mps_data_type_int16:
       return MPSDataTypeInt16;
     case DataType::mps_data_type_int32: