Wrong BlenderbotConfig description (max_position_embeddings)
Hi there, the documentation page for BlenderbotConfiguration has a wrong parameter description
[https://huggingface.co/transformers/model_doc/blenderbot.html](url)

max_position_embeddings (int, optional, defaults to 1024) – The maximum sequence length that this model might ever be used with. Typically set this to something large just in case (e.g., 512 or 1024 or 2048).

But it's actually 128, as shown in the source code:
[https://huggingface.co/transformers/_modules/transformers/models/blenderbot/configuration_blenderbot.html#BlenderbotConfig](url)

def __init__( self, vocab_size=8008, max_position_embeddings=128, encoder_layers=2, encoder_ffn_dim=10240, ...

And by the way, do anyone know how to increase the maximum sequence length of this model? If I modify the config.max_position_embeddings, it will result in an error: (BlenderbotForConditionalGeneration)

`size mismatch for model.encoder.embed_positions.weight: copying a param with shape torch.Size([128, 2560]) from checkpoint, the shape in current model is torch.Size([1024, 2560]).`
`size mismatch for model.decoder.embed_positions.weight: copying a param with shape torch.Size([128, 2560]) from checkpoint, the shape in current model is torch.Size([1024, 2560]).`

With the length of 128 tokens, it will "forget" the conversation's topic quite fast, since the input has to be trimmed.

Thanks in advance.

@patrickvonplaten @patil-suraj

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