Fix doc examples: modify config before super().__init__
# What does this PR do?

Some PT Causal LM models modify config after `super().__init__(config)`. This causes some doc examples failed.
See #14672 for the details.

This PR fixes this issue: It does the same as what has been done in 
```
class ProphetNetForCausalLM(ProphetNetPreTrainedModel):
    def __init__(self, config):
        # set config for CLM
        config = copy.deepcopy(config)
        config.is_decoder = True
        config.is_encoder_decoder = False
        super().__init__(config)
        self.prophetnet = ProphetNetDecoderWrapper(config)
```

Fixes #14672

## Who can review?
@patrickvonplaten 

 This makes sense to me - thanks for outlining the problem in the issue! @sgugger @LysandreJik - can you take a quick look as well?