DETR: changing position_embedding and key_value_position_embedding args

# What does this PR do?

This PR refers to #19833 , and it just update some variables/docstrings names. Quoting the Issue, the paper mentions that the `position_embeddings` argument of the cross-attention layer are these input embeddings called `object queries`. And the `key_value_position_embeddings` is refered to as `spatial_position_embeddings`.

This PR is limited to DETR model. 

### Notes

This is my first contribution, so I'm happy to adjust anything in this PR. I ran all tests and style, and it went all, except for one:
`make fixup`. I got the following output:

![image](https://user-images.githubusercontent.com/70359945/235523582-4a0dded1-abb7-4ada-b673-f59b9542dc4a.png)

Reading the output, I assume it is about other file using classes in modeling_detr. I'll wait for updates. I will also wait for review for doc updating or more guidance.

Fixes # (issue)

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
https://github.com/huggingface/transformers/issues/19833
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@NielsRogge 
@amyeroberts 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23091). All of your documentation changes will be reflected on that endpoint.<|||||>Hi,

I dont know if this issue is still up.

I believe you need to change the names of the files mentioned in the fixup too. Since in the paper of [Conditional DETR](https://arxiv.org/pdf/2108.06152.pdf), they also use the same nomenclature (sometimes  `object queries` are also called  `content queries` though) .

For example in [modeling_conditional_detr.py](https://github.com/huggingface/transformers/blob/bd469c40659ce76c81f69c7726759d249b4aef49/src/transformers/models/conditional_detr/modeling_conditional_detr.py#LL556C9-L556C28) the names of the forward function are still `position_embeddings`, so you would need to change that to `object queries` for consistency.

Same applies to the other file mentioned in the fixup too. I am also new to fixing PRs in this repo, so I would leave this decision to the reviewers, but I believe it makes sense if you would like to apply the changes @Lorenzobattistela . If not, maybe another issue could be created for that.
<|||||>@A-guridi Hey, I understand these names could be changed to keep consistency, and I am up to do this. But I don't know if this is the right to do since the issue is specific about DETR. But I'll try what you said, let's wait up the reviewers
<|||||>@NielsRogge Could you review and confirm if it aligns with your suggestion in #19833? <|||||>> Thanks for working on this, left some comments.
> 
> Specifically, DETR's decoder uses 2 types of position embeddings:
> 
> * the ones that are added to the inputs i.e. hidden states of each cross-attention layer (the object_queries)
> * the ones that are added to the keys and values of each cross-attention layer (the spatial_position_embeddings)

working on it<|||||>git history got messed up, will open a new PR just with the correct changes<|||||>Reopened PR #24652