Fix tests imports dpr
There were changes in the locations of some functions used for tests in #5350.
When #5279 was merged some tests couldn't run.

I fixed the imports of those functions.
When I re-ran the tests I noticed that some didn't pass for the DPRReaderTokenizer so I fixed them.

Some tests are failing, please only merge once everything pass.<|||||>Yep I'm on it<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=h1) Report
> Merging [#5576](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d2a93991158f15993eba9ab421d82766b892f948&el=desc) will **increase** coverage by `0.14%`.
> The diff coverage is `74.31%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5576/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5576      +/-   ##
==========================================
+ Coverage   76.84%   76.99%   +0.14%     
==========================================
  Files         141      145       +4     
  Lines       24685    25049     +364     
==========================================
+ Hits        18969    19286     +317     
- Misses       5716     5763      +47     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.65% <ø> (ø)` | |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <ø> (ø)` | |
| [src/transformers/data/datasets/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL3NxdWFkLnB5) | `47.56% <47.56%> (ø)` | |
| [src/transformers/tokenization\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZHByLnB5) | `57.65% <57.65%> (ø)` | |
| [src/transformers/modeling\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kcHIucHk=) | `97.45% <97.45%> (ø)` | |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.24% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/configuration\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2Rwci5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/data/datasets/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| ... and [5 more](https://codecov.io/gh/huggingface/transformers/pull/5576/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=footer). Last update [e49393c...878b09c](https://codecov.io/gh/huggingface/transformers/pull/5576?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>All green, merging