fix: #14486 do not use BertPooler in DPR
Author: PaulLerner
Pull request: yes

# What does this PR do?

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Fixes #14486


## 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/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@patrickvonplaten


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## What did I do?
Fixed the lines we discussed about.
`pytest tests/test_modeling_dpr.py` gives the same output before and after:
```
============================= test session starts ==============================
platform linux -- Python 3.7.11, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /people/lerner/code/transformers, configfile: setup.cfg
plugins: forked-1.4.0, xdist-2.5.0, dash-2.0.0, timeout-2.0.2, hypothesis-6.34.2
collected 52 items

tests/test_modeling_dpr.py .....ss...............s..ssss.s.............. [ 86%]
sss..ss                                                                  [100%]

=============================== warnings summary ===============================
../../.local/lib/python3.7/site-packages/pandas/util/testing.py:20
  /people/lerner/.local/lib/python3.7/site-packages/pandas/util/testing.py:20: DeprecationWarning: `np.bool` is a deprecated alias for the builtin `bool`. To silence this warning, use `bool` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.bool_` here.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    from pandas._libs import testing as _testing

../../../../vol/work/lerner/anaconda3/envs/transformers/lib/python3.7/site-packages/flatbuffers/compat.py:19
  /vol/work/lerner/anaconda3/envs/transformers/lib/python3.7/site-packages/flatbuffers/compat.py:19: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

-- Docs: https://docs.pytest.org/en/stable/warnings.html
================== 39 passed, 13 skipped, 2 warnings in 4.10s ==================
```

Ran all the other stuff as instructed in CONTRIBUTING:
```
$ make fixup
Checking/fixing src/transformers/models/dpr/modeling_dpr.py
All done! ✨ 🍰 ✨
1 file left unchanged.
python utils/custom_init_isort.py
python utils/style_doc.py src/transformers docs/source --max_len 119
running deps_table_update
updating src/transformers/dependency_versions_table.py
python utils/check_copies.py
python utils/check_table.py
python utils/check_dummies.py
python utils/check_repo.py
Checking all models are included.
Checking all models are public.
2022-01-07 13:53:00.515988: W tensorflow/stream_executor/platform/default/dso_loader.cc:64] Could not load dynamic library 'libcudart.so.11.0'; dlerror: libcudart.so.11.0: cannot open shared object file: No such file or directory; LD_LIBRARY_PATH: /usr/local/cuda/lib64
Checking all models are properly tested.
Checking all objects are properly documented.
Checking all models are in at least one auto class.
python utils/check_inits.py
python utils/tests_fetcher.py --sanity_check
$ make quality
black --check examples tests src utils
All done! ✨ 🍰 ✨
1231 files would be left unchanged.
isort --check-only examples tests src utils
Skipped 1 files
python utils/custom_init_isort.py --check_only
flake8 examples tests src utils
python utils/style_doc.py src/transformers docs/source --max_len 119 --check_only
$ make repo-consistency
python utils/check_copies.py
python utils/check_table.py
python utils/check_dummies.py
python utils/check_repo.py
Checking all models are included.
Checking all models are public.
2022-01-07 13:54:31.057343: W tensorflow/stream_executor/platform/default/dso_loader.cc:64] Could not load dynamic library 'libcudart.so.11.0'; dlerror: libcudart.so.11.0: cannot open shared object file: No such file or directory; LD_LIBRARY_PATH: /usr/local/cuda/lib64
Checking all models are properly tested.
Checking all objects are properly documented.
Checking all models are in at least one auto class.
python utils/check_inits.py
python utils/tests_fetcher.py --sanity_check
```

Comments:
@PaulLerner - thanks a lot for your PR - I took the liberty to fix the final failing tests (hope that was ok)

Hi, you welcome, sorry I didn’t notice that some test had failed!