Error when using np.where() during squad tokenization 
# 🐛 Bug

## Information

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

Language I am using the model on (English, Chinese ...):English

The problem arises when using:
* [x] the official example scripts: squad1.1
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: SQUaD task

* [ ] my own task or dataset: (give details below)

## To reproduce
 I'm confused about this line:
https://github.com/huggingface/transformers/blob/896300177bf9f35feac4698370212a80a5ab6138/src/transformers/data/processors/squad.py#L230

Should this be  
```
pad_token_indices = np.where(np.array(span["input_ids"]) == tokenizer.pad_token_id)
```
Cause "p_mask: mask with 1 for token than cannot be in the answer (0 for token which can be in an answer)", and the padding positions should not be answers

Steps to reproduce the behavior:
```python
span["input_ids"] = [1,1,3,0,0,0]
np.where([1,1,3,0,0,0] == tokenizer.pad_token_id)
# return (array([], dtype=int64),)

np.where(np.array([1,1,3,0,0,0]) == tokenizer.pad_token_id)
# return (array([3, 4, 5]),)
```

<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     Important! Use code tags to correctly format your code. See https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks#syntax-highlighting
     Do not use screenshots, as they are hard to read and (more importantly) don't allow others to copy-and-paste your code.-->

## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
As described above.

## 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: 7a68d401388bc68f10dfeb591709352736a6c0b6
- Platform:
- Python version: python3.6.8
- PyTorch version (GPU?):1.5.0 CPU
- Tensorflow version (GPU?):
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?:NO

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.
