Tokenizer: UnboundLocalError with PaddingStrategy MAX_LENGTH
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
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- `transformers` version: 3.1.0
- Platform: macOS-10.15.7-x86_64-i386-64bit
- Python version: 3.8.5
- PyTorch version (GPU?): 1.6.0 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no


### Who can help
 
 tokenizers: @mfuntowicz

## Information

Model I am using: <transformers.tokenization_bert.BertTokenizer>

The problem arises when using:
* the official example scripts: 
using the `encode_plus`

The tasks I am working on is:
* Tokenizing

## To reproduce

Steps to reproduce the behavior:

1. tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")
2. tokenizer.encode_plus("hello word", max_length=128, padding=PaddingStrategy.MAX_LENGTH)

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`UnboundLocalError: local variable 'padding_strategy' referenced before assignment`

## Expected behavior

Return Tokenizer output