Position embedding in the DETR model
### System Info

According to the argument definition of the `DetrDecoderLayer.forward()` specified here:
https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/models/detr/modeling_detr.py#L723-L728

The `positional_embeddings` argument for the cross-attention should be assigned by the `position_embeddings` variable instead of `query_position_embeddings `.
https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/models/detr/modeling_detr.py#L757-L764

Is this an error in the argument definition or the code part? 

Thank you! 

### Who can help?

@NielsRogge

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

It is from the transformers code.

Arguments definition:
https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/models/detr/modeling_detr.py#L723-L728

Cross-attention code:
https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/models/detr/modeling_detr.py#L757-L764

### Expected behavior

Either:
1. The `positional_embeddings` argument for the cross-attention should be assigned by the `position_embeddings` variable instead of `query_position_embeddings `, or
2. Update the documentation of the argument to the correct one.
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/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hey @NielsRogge , could you explain how to solve this issue? You just put the Goo first issue label on it but it's not clear what a contributor would have to do to fix it.<|||||>Hi @NielsRogge, I would like to take on this.
As Sylvain suggested, could you offer some context on how to go with this?
Thanks :)<|||||>Yeah I marked this as good first issue as someone could take a deeper dive into DETR's position embeddings.

Reading the [paper](https://arxiv.org/abs/2005.12872) for that could definitely be helpful. But the implementation is correct, it's probably internal variables/docstrings that need to be updated. From the paper:

> Since the decoder is also permutation-invariant, the N input embeddings must be different to produce different results. These input embeddings are learnt positional encodings that we refer to as object queries, and similarly to the encoder, we add them to the input of each attention layer.

So the `position_embeddings` argument of the cross-attention layer are exactly these input embeddings, often also called "content embeddings" or "object queries".

Then a bit later on in the paper they state:

> There are two kinds of positional encodings in our model: spatial positional encodings and output positional encodings (object queries).

So the `key_value_position_embeddings` arguments of the cross-attention layer refer to these spatial position encodings. These are added to the keys and values in the cross-attention operation. 

So we could for clarity update the "position_embeddings" argument to "object_queries", and the "key_value_position_embeddings" argument to "spatial_position_embeddings"<|||||>Hello @daspartho @NielsRogge , wanted to inquire as to whether any progress was made on this? I'd like to take a look.<|||||>Hello @NielsRogge , I am currently working on this issue. I've read the article and I do understand what has to be changed. My question is if we only have to change the `DetrDecoderLayer` class (in the respective `forward` function mentioned above or al position_embeddings args have to change too. 

I did some local tests too, and noted that changing only in the function forward i mentioned to `object_queries` and `spatial_position_embeddings`, many tests broke because of wrong arguments passed since names changed. In order to change these arguments, we need to change them in tests? 

I looked up some tests, but I do think the problem is in the code itself, since classes related to that one would be passing arguments wrongly. 

This is my first contribution to an open source project this size, and I'm really happy to do it. Thanks in advance.<|||||>Hey @NielsRogge is this issue still open? If yes can I take this?<|||||>Hey @hackpk I'm finishing touches in my PR to fix this Issue, so Idk...<|||||>That's great.I'll look for another issue then. Thanks. <|||||>No problem, good luck :D