Padding Strategy Code missing an else case (maybe?)
## Environment info
- `transformers` version: 3.0.2
- Platform: macOS 10.15.5 
- Python version: 3.7
- PyTorch version (GPU?): 1.5 GPU-Yes
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
 tokenizers: @mfuntowicz
 Summarization: @sshleifer
 T5: @patrickvonplaten
 
## Information

Model I am using (T5 via Autotokenizer):

The problem arises when using:
`tokenizer([line], max_length=max_length, padding='max_length' if pad_to_max_length else False,
                     truncation=True, return_tensors=return_tensors, **extra_kw)`

In batch encoding, the latest code decides on a padding strategy:
`_get_padding_truncation_strategies(
        self, padding=False, truncation=False, max_length=None, pad_to_multiple_of=None, verbose=True, **kwargs
    ):`

       ` elif padding is not False:
            if padding is True:
                padding_strategy = PaddingStrategy.LONGEST  # Default to pad to the longest sequence in the batch
            elif not isinstance(padding, PaddingStrategy):
                padding_strategy = PaddingStrategy(padding)`

While calling the tokenizer, instead of 'max_length' I first gave the actual PaddingStrategy.MAX_LENGTH Enum as argument,
but the above code throws an error as 'padding_strategy' is not defined.
## To reproduce
Call the tokenizer as:
`tokenizer([line], max_length=max_length, padding=PaddingStrategy.MAX_LENGTH if pad_to_max_length else False,
                     truncation=True, return_tensors=return_tensors, **extra_kw)`

## Expected behavior

The PaddingStrategy enum should be assigned no issue.

##Suggested Solution
               
                    ` elif padding is not False:
                              if padding is True:
                                 padding_strategy = PaddingStrategy.LONGEST  # Default to pad to the longest sequence in the batch
                      elif not isinstance(padding, PaddingStrategy):
                             padding_strategy = PaddingStrategy(padding)
                      else:
                          padding_strategy = padding`        
It's a one line fix basically, I can raise a PR for the same, unless PaddingStrategy wasn't designed to be used directly?

This issue also applies to the `truncation` parameter.

I assumed the enums are supposed to be used directly because the release notes (https://github.com/huggingface/transformers/releases/tag/v3.0.0) explicitly mention the `TensorType` enum, which is defined right below the `PaddingStrategy` and `TruncationStrategy` enums.

I agree that this is a problem that should be fixed, if the enums are meant to be used.<|||||>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.
<|||||>I think we should fix this, wdyt @LysandreJik ?<|||||>I believe this was already fixed by https://github.com/huggingface/transformers/pull/7610<|||||>Nice, thanks!