Can't set attention_probs_dropout_prob in LEDConfig
## Environment info

- `transformers` version: 4.9.0
- Python version: 3.8.10
- PyTorch version (GPU?): 1.9.0


### Who can help
@patrickvonplaten @beltagy 


## Information

Loading LEDForConditionalGeneration throws an error on line 314 of configuration_utils.py:
```
"Can't set attention_probs_dropout_prob with value 0.1 for LEDConfig"
```
but this parameter is required in line 149 of [modeling_led.py](https://github.com/huggingface/transformers/blob/master/src/transformers/models/led/modeling_led.py)
```
classLEDEncoderSelfAttention(nn.Module):
...
   self.dropout=config.attention_probs_dropout_prob
```
It works fine if I remove it from the config.


## To reproduce

I am trying to load a LEDForConditionalGeneration, converted from a Bart model.
See [convert_bart_to_longformerencoderdecoder.py](https://github.com/allenai/longformer/blob/master/scripts/convert_bart_to_longformerencoderdecoder.py) and some hints on how to [replace LongformerEncoderDecoderForConditionalGeneration with LEDForConditionalGeneration](https://github.com/allenai/longformer/issues/192)


The reason you're getting this error is because `attention_probs_dropout_prob` is (as of now) only defined as a getter, not as a setter, as you can see [here](https://github.com/huggingface/transformers/blob/ba15fe7995a02357ecea6e7024918f6915564c36/src/transformers/models/led/configuration_led.py#L173-L175). The reason for this is that some models call this dropout value "attention_dropout", while others call it "attention_probs_dropout_prob". To ensure you can also access it with the different name, this property was defined. 

For now, you can get the attention dropout by calling `config.attention_probs_dropout_prob`, but not set it. You can only set it using `config.attention_dropout`.

However, @nreimers is currently working on adding setters (besides getters) for all attribute names and their aliases. Expect a fix for this in the near future.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.