ProphetNetForConditionalGeneration model isn't returning all objects properly
## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.6.1
- Platform: Google Colab
- Python version: 
- PyTorch version (GPU?): GPU
- Tensorflow version (GPU?): 
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
- text generation: @patrickvonplaten



## Information

The model I am using Prophetnet:

The problem arises when using:
* my own modified scripts: [My notebook](https://colab.research.google.com/drive/1rmZyTXsdEDDpx8tbX-Gt6Uj9NuQi92VK?usp=sharing)

The tasks I am working on is:
*  an official SQUaD task: Question Generation

## To reproduce

Steps to reproduce the behavior:

1. Just run the notebook 
2. After running a single inference I am only getting 4 objects while I should get loss and other objects.


## Expected behavior

After running a single inference I am only getting 4 objects while I should get loss and other objects.  @patrickvonplaten


What are all the objects you expect to get? The `loss` is only returned if you pass the labels to the model - otherwise it cannot compute any loss. Please check out the [return statement of ProphetNetForConditionalGeneration's forward method for more information](https://huggingface.co/transformers/model_doc/prophetnet.html#transformers.ProphetNetForConditionalGeneration.forward). <|||||>Thank you, it worked @LysandreJik 