fix doc example - AssertionError: has to be configured as a decoder. (#15124)

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/models/prophetnet/modeling_prophetnet.py

@@ -1471,7 +1471,6 @@ class ProphetNetDecoder(ProphetNetPreTrainedModel):
 
         >>> tokenizer = ProphetNetTokenizer.from_pretrained("microsoft/prophetnet-large-uncased")
         >>> model = ProphetNetDecoder.from_pretrained("microsoft/prophetnet-large-uncased", add_cross_attention=False)
-        >>> assert model.config.is_decoder, f"{model.__class__} has to be configured as a decoder."
         >>> inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
         >>> outputs = model(**inputs)