

TITLE
ImportError: cannot import name 'DPRReader' from 'transformers'

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Hi, I am trying to run below code, it can be found at this [link](https://huggingface.co/transformers/model_doc/dpr.html#dprreader)
```
from transformers import DPRReader, DPRReaderTokenizer
tokenizer = DPRReaderTokenizer.from_pretrained('facebook/dpr-reader-single-nq-base')
model = DPRReader.from_pretrained('facebook/dpr-reader-single-nq-base')
encoded_inputs = tokenizer(
        questions=["What is love ?"],
        titles=["Haddaway"],
        texts=["'What Is Love' is a song recorded by the artist Haddaway"],
        return_tensors='pt'
    )
outputs = model(**encoded_inputs)
start_logits = outputs.stat_logits
end_logits = outputs.end_logits
relevance_logits = outputs.relevance_logits
```
But got error

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.0.2
- Platform: Windows (GCP Instance)
- Python version: 3.8.6
- PyTorch version (GPU?): '1.7.0+cpu'
- Tensorflow version (GPU?): Not installed
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
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 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten @TevenLeScao
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 -->

## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [x] 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)
* [x] my own task or dataset: (give details below)
I am just trying to run DPR model. 

## To reproduce

Steps to reproduce the behavior:

1. Execute this in your python
```
from transformers import DPRReader, DPRReaderTokenizer
tokenizer = DPRReaderTokenizer.from_pretrained('facebook/dpr-reader-single-nq-base')
model = DPRReader.from_pretrained('facebook/dpr-reader-single-nq-base')
encoded_inputs = tokenizer(
        questions=["What is love ?"],
        titles=["Haddaway"],
        texts=["'What Is Love' is a song recorded by the artist Haddaway"],
        return_tensors='pt'
    )
outputs = model(**encoded_inputs)
start_logits = outputs.stat_logits
end_logits = outputs.end_logits
relevance_logits = outputs.relevance_logits
```

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The error traceback is below:
```
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-2f646c4dd41f> in <module>
----> 1 from transformers import DPRReader, DPRReaderTokenizer
      2 tokenizer = DPRReaderTokenizer.from_pretrained('facebook/dpr-reader-single-nq-base')
      3 model = DPRReader.from_pretrained('facebook/dpr-reader-single-nq-base')
      4 encoded_inputs = tokenizer(
      5         questions=["What is love ?"],

ImportError: cannot import name 'DPRReader' from 'transformers' (C:\<some_path>\env\lib\site-packages\transformers\__init__.py)

```

## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
I haven't executed this but I would hope nothing in output as all the results are stored in variables.