[data][tests-only] Fix nightly microbenchmark for read_images #39609

Fix a test error in read_images_comparison_microbenchmark_single_node.

Signed-off-by: Stephanie Wang <swang@cs.berkeley.edu>

release/nightly_tests/dataset/image_loader_microbenchmark.py

@@ -530,6 +530,7 @@ def __getitem__(self, idx: int) -> Any:
             ray_dataset = ray.data.read_datasource(
                 mds_source, paths=args.mosaic_data_root
             )
+            ray_dataset = ray_dataset.map(crop_and_flip_image)
             for i in range(args.num_epochs):
                 iterate(
                     ray_dataset.iter_torch_batches(batch_size=args.batch_size),