DPR pooler weights not loading correctly
### System Info

tested on multiple versions

- `transformers` version: 4.12.3
- Platform: Linux-4.14.281-212.502.amzn2.x86_64-x86_64-with-glibc2.10
- Python version: 3.8.10
- PyTorch version (GPU?): 1.11.0+cu102 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?:no


another environment
- `transformers` version: 4.16.2
- Platform: macOS-12.6-x86_64-i386-64bit
- Python version: 3.9.7
- PyTorch version (GPU?): 1.11.0+cu102 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: no

### Who can help?

@patrickvonplaten @lhoestq 



### Information

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

### Tasks

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

### Reproduction

```
from transformers import DPRContextEncoder, DPRQuestionEncoder
question_encoder_path = "facebook/dpr-question_encoder-single-nq-base"  # can also be a custom checkpoint
answer_encoder_path = "facebook/dpr-ctx_encoder-single-nq-base"

DPRQuestionEncoder.from_pretrained(question_encoder_path)
DPRContextEncoder.from_pretrained(answer_encoder_path)
```

results in the following message
```
Some weights of the model checkpoint at facebook/dpr-question_encoder-single-nq-base were not used when initializing DPRQuestionEncoder: ['question_encoder.bert_model.pooler.dense.weight', 'question_encoder.bert_model.pooler.dense.bias']
- This IS expected if you are initializing DPRQuestionEncoder from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPreTraining model).
- This IS NOT expected if you are initializing DPRQuestionEncoder from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).

Some weights of the model checkpoint at facebook/dpr-ctx_encoder-single-nq-base were not used when initializing DPRContextEncoder: ['ctx_encoder.bert_model.pooler.dense.weight', 'ctx_encoder.bert_model.pooler.dense.bias']
- This IS expected if you are initializing DPRContextEncoder from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPreTraining model).
- This IS NOT expected if you are initializing DPRContextEncoder from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).
```

### Expected behavior

Model loads successfully without re-intitializing weights
@ArthurZucker could you take a look here (happy to answer questions about the model)<|||||>on it! <|||||>Hi @ArthurZucker , do you have any updates on this by chance? I'm getting the same issue, but the results I get when benchmarking do not suggest random initialization of the weights.<|||||>Hey, it seems that the issue comes from the following [line](https://github.com/huggingface/transformers/blob/main/src/transformers/models/dpr/modeling_dpr.py#L178), where it is hardcoded that `add_pooling_layer=False`. Setting it to `True` fixes the issue. Now I am not very familiar with the model, but a few test seems to be link to that checkpoint. Let me open a PR for a fix ! <|||||>Hey! So as mentioned here in a previous [PR](https://github.com/huggingface/transformers/pull/15068/commits/95eaf44ce93bbf46b552c606957f98b354dae73a), the optional pooling layer was removed as no checkpoints use it. 

My first question would then be : do you need to have the `BERTPoolerLayer`? It is a bit confusing indeed that the pooling output do not come from the `BertPoolerLayer`. Have a look at #14486, I think it explains pretty well what's going on here. 

We have to ways to go about this : 
1. We add an argument in the config of DPR, and take care about updating the online config to have no breaking changes. 
2. If you don't need it, then we just add a warning/update the online weights doing `from_pretrained` then `push_to_hub`  and the checkpoints will then not include the `pooler` weights 😄 <|||||>Hi Arthur, I think I understand. Thanks for getting back so quickly! Its
performance suggests that the model is loading correctly so that must be
it! Thanks!

On Thu, Oct 20, 2022 at 3:42 AM Arthur ***@***.***> wrote:

> Hey! So as mentioned here in a previous PR
> <https://github.com/huggingface/transformers/pull/15068/commits/95eaf44ce93bbf46b552c606957f98b354dae73a>,
> the optional pooling layer was removed as no checkpoints use it.
>
> My first question would then be : do you need to have the BERTPoolerLayer?
> It is a bit confusing indeed that the pooling output do not come from the
> BertPoolerLayer. Have a look at #14486
> <https://github.com/huggingface/transformers/issues/14486>, I think it
> explains pretty well what's going on here.
>
> We have to ways to go about this :
>
>    1. We add an argument in the config of DPR, and take care about
>    updating the online config to have no breaking changes.
>    2. If you don't need it, then we just add a warning/update the online
>    weights doing from_pretrained then push_to_hub and the checkpoints
>    will then not include the pooler weights 😄
>
> —
> Reply to this email directly, view it on GitHub
> <https://github.com/huggingface/transformers/issues/19111#issuecomment-1285084844>,
> or unsubscribe
> <https://github.com/notifications/unsubscribe-auth/AXTXAGMEKV3HT2J3QSWTHF3WEDZVZANCNFSM6AAAAAAQQLMVMQ>
> .
> You are receiving this because you commented.Message ID:
> ***@***.***>
>
