Remove task arg in load_dataset in image-classification example (#28408)

* Remove `task` arg in `load_dataset` in image-classification example

* Manage case where "train" is not in dataset

* Add new args to manage image and label column names

* Similar to audio-classification example

* Fix README

* Update tests

examples/pytorch/test_accelerate_examples.py

@@ -322,6 +322,7 @@ class ExamplesTestsNoTrainer(TestCasePlus):
             --output_dir {tmp_dir}
             --with_tracking
             --checkpointing_steps 1
+            --label_column_name labels
         """.split()
 
         run_command(self._launch_args + testargs)