auxiliary_loss works for Deformable Detr
Author: long8v
Pull request: yes

# What does this PR do?
DeformableDetr does not work when auxiliary_loss=True.
Since Deformable Detr has list of class_embed, bbox_embed, this code will raise NotImplementedError.
```python
                intermediate = outputs.intermediate_hidden_states if return_dict else outputs[4]
                outputs_class = self.class_embed(intermediate)
                outputs_coord = self.bbox_embed(intermediate).sigmoid()
```
```python
outputs_class = self.class_embed(intermediate)
File "/opt/conda/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1110, in _call_impl
return forward_call(*input, **kwargs)
File "/opt/conda/lib/python3.8/site-packages/torch/nn/modules/module.py", line 201, in _forward_unimplemented
raise NotImplementedError
NotImplementedError
```
To fix this, we can simply use predefined `outputs_class` and `outputs_coord` in this [line](https://github.com/huggingface/transformers/blob/main/src/transformers/models/deformable_detr/modeling_deformable_detr.py#L1942-L1943).

## Before submitting
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## Who can review?

@NielsRogge 

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