[DPR] fix unexpected "pooler" keys
# What does this PR do?
Fixes #19111, by adding  [r"pooler"] to the list of ignored unexpected keys for both the `DPRPretrainedContextEncoder` and `DPRPretrainedQuestionEncoder`. 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20210). All of your documentation changes will be reflected on that endpoint.