Result changes if we don't pass attension mask in TFDistilbert model on SQUADv1 dataset
## Environment info
- `transformers` version: latest
- Platform: Colab
- Python version: 
- Tensorflow version (GPU?): 2.3.0
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
I used the below code for getting Model
```
from transformers import  AutoTokenizer, TFAutoModelForQuestionAnswering
tokenizer = AutoTokenizer.from_pretrained('distilbert-base-uncased-distilled-squad')
model =  TFAutoModelForQuestionAnswering.from_pretrained('distilbert-base-uncased-distilled-squad', return_dict=True)
```
 tokenizers: @mfuntowicz
 examples/seq2seq: @patil-suraj
 tensorflow: @jplu 

## Information

The model I am using **TFDistilbert** pretrained.

The problem arises when using:
* my own modified scripts:
This is the Notebook
[Colab](https://github.com/bhadreshpsavani/UnderstandingNLP/blob/master/TFLiteExperimentsQALatest.ipynb)

The tasks I am working on is:
*  an official SQUaD task

## To reproduce

Steps to reproduce the behavior: [Colab](https://github.com/bhadreshpsavani/UnderstandingNLP/blob/master/TFLiteExperimentsQALatest.ipynb)

## Expected behavior

The performance should be the same because the Attention mask is the optional argument, if we don't pass it will create it internally. 

With Attention Mask:
```
OrderedDict([('exact', 77.71050141912077),
           ('f1', 85.5370981182013),
           ('total', 10570)])
```
Without Attention Mask:
```
OrderedDict([('exact', 72.82876064333927),
             ('f1', 80.71521545953475),
             ('total', 10570)])
```
     

`attention_mask` is an optional argument, but that doesn't mean that it should not be passed to the function. If `attention_mask` is `None` then it is initialized to attend all tokens (all 1's in the `attention_mask` tensor), which is incorrect if the input is a batch that includes padding tokens. => It's better to simply pass the `attention_mask` to the forward function.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>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.