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_pytorch_examples.py

@@ -398,6 +398,7 @@ class ExamplesTests(TestCasePlus):
             --max_steps 10
             --train_val_split 0.1
             --seed 42
+            --label_column_name labels
         """.split()
 
         if is_torch_fp16_available_on_device(torch_device):