Issue while using DPR with tensorflow and py torch
## 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: 4.6.1
- Platform:Windows
- Python version:3.6
- PyTorch version (GPU?No):1.7.0
- Tensorflow version (GPU? No):2.3.1
- Using GPU in script?:No
- Using distributed or parallel set-up in script?:No

### Who can help
Hi @LysandreJik , @Rocketknight1 ,@patrickvonplaten, @lhoestq ,@sgugger

I'm trying the basic example mentioned in the DPR documentation but facing the below issue,
can you please help me,and can you please let me know using what functionality I can get the answer for a given question to the provided text, as `relevance_logits` is just giving a number which I'm not understanding the significance.

Thanks you all!!

```
#########################################################################

##Using **pt** as return tensors
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
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-86e12c26a869> in <module>
      9  )
     10 outputs = model(**encoded_inputs)
---> 11 start_logits = outputs.stat_logits
     12 end_logits = outputs.end_logits
     13 relevance_logits = outputs.relevance_logits

AttributeError: 'DPRReaderOutput' object has no attribute 'stat_logits' 

#########################################################################

##Using tf as return_tensors
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='tf'
 )
outputs = model(**encoded_inputs)
start_logits = outputs.stat_logits
end_logits = outputs.end_logits
relevance_logits = outputs.relevance_logits

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-6f88caac8403> in <module>
      8      return_tensors='tf'
      9  )
---> 10 outputs = model(**encoded_inputs)
     11 start_logits = outputs.stat_logits
     12 end_logits = outputs.end_logits

~\Anaconda3\lib\site-packages\torch\nn\modules\module.py in _call_impl(self, *input, **kwargs)
    725             result = self._slow_forward(*input, **kwargs)
    726         else:
--> 727             result = self.forward(*input, **kwargs)
    728         for hook in itertools.chain(
    729                 _global_forward_hooks.values(),

~\Anaconda3\lib\site-packages\transformers\models\dpr\modeling_dpr.py in forward(self, input_ids, attention_mask, inputs_embeds, output_attentions, output_hidden_states, return_dict)
    641             raise ValueError("You cannot specify both input_ids and inputs_embeds at the same time")
    642         elif input_ids is not None:
--> 643             input_shape = input_ids.size()
    644         elif inputs_embeds is not None:
    645             input_shape = inputs_embeds.size()[:-1]

AttributeError: 'tensorflow.python.framework.ops.EagerTensor' object has no attribute 'size'

```



Hey @MaheshChandrra, 

It seems like there is a type as it should be called `outputs.start_logits` instead of `outputs.stat_logits`. Would you like to open a PR to fix it? :-)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.