RagRetriever question_hidden_states shape
## Environment info
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- `transformers` version: 4.2.2
- Platform: Linux-5.4.0-60-generic-x86_64-with-debian-bullseye-sid
- Python version: 3.7.7
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): 2.3.0 (True)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?:no

### Who can help
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 RAG: @patrickvonplaten, @lhoestq

## Information

The shape of RagRetriever's argument question_hidden_states mentioned in the document is `(batch_size, vector_size)`.
However, when RagModel calls self.retriever in forward function, it passes question_encoder_last_hidden_state`(batch_size, seq_len, vector_size)` to RagRetriever. Maybe question_encoder_last_hidden_state should be replaced with question_encoder_pooler_output?

The problem arises when using:
* [v] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [v] my own task or dataset: (give details below)

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## Expected behavior

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maybe a small bug?
I'm pretty sure it's just a bad naming of that variable. It should be question_encoder_pooler_output.

Indeed this variable is computed as the first output of a `DPRQuestionEncoder`. The DPR encoders don't return the hidden states (they basically skip it) to return the pooler output (aka DPR embeddings) of shape `(batch_size, vector_size)` instead.

If you want to contribute, feel free to open a PR to rename this variable :) <|||||>This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

If you think this still needs to be addressed please comment on this thread.