[fbsync] fix bug when using PIL backend in references/classification (#7665)

Summary: Co-authored-by: Max Chuprov <[email protected]>

Reviewed By: vmoens

Differential Revision: D46724127

fbshipit-source-id: 300232aadc8fe22ece46c229ec59ac0b95784756

Author: NicolasHug

references/classification/presets.py

@@ -69,11 +69,10 @@ def __init__(
         backend="pil",
     ):
         trans = []
-
         backend = backend.lower()
         if backend == "tensor":
             trans.append(transforms.PILToTensor())
-        else:
+        elif backend != "pil":
             raise ValueError(f"backend can be 'tensor' or 'pil', but got {backend}")
 
         trans += [