Fix shadowing error in mmap_data_loader.cpp (pytorch#11858)

### Summary
Fix a -Wshadow error causing internal build failures. I will add this to
the GitHub CI, as well.

### Test plan
CI

Author: GregoryComer

extension/data_loader/mmap_data_loader.cpp

@@ -181,9 +181,9 @@ Result<FreeableBuffer> MmapDataLoader::load(
     size_t size,
     ET_UNUSED const DataLoader::SegmentInfo& segment_info) const {
   // Ensure read range is valid.
-  auto err = validate_input(offset, size);
-  if (err != Error::Ok) {
-    return err;
+  auto validation_err = validate_input(offset, size);
+  if (validation_err != Error::Ok) {
+    return validation_err;
   }
 
   // mmap() will fail if the size is zero.