TOKENIZER: truncation not working for batch
## Environment info

- `transformers` version: 2.11.0
- Platform: Linux-4.15.0-106-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.4
- PyTorch version (GPU?): 1.5.1 (False)
- Tensorflow version (GPU?): 2.2.0 (False)
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

### Who can help
 
 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 examples/distillation: @VictorSanh

## Information

Model I am using: DistilBertForSequenceClassification.

The tokenizer does not truncate when I pass a list of strings. It only works when I pass a single string.

## To reproduce

Copy/paste (or just read) code below.  

```python
from transformers import DistilBertTokenizer

tokenizer = DistilBertTokenizer.from_pretrained("distilbert-base-uncased")

sentence = "Submit a bug report to help us improve transformers."
output = tokenizer(sentence, padding=True, truncation=True, max_length=4)

print(output['input_ids'])
# 4 tokens as expected from *max_length=4*
# out: [101, 12040, 1037, 102]

# now let's test with multiple sentences
sentences = [
  "Submit a bug report to help us improve transformers.", 
  "Benchmark a part of this library and share your results"
] 

output = tokenizer(sentences, padding=True, truncation=True, max_length=4)
print(output['input_ids'])
# output is returning all tokens, it is not truncating to max_length!
# out: [[101, 12040, 1037, 11829, 3189, 2000, 2393, 2149, 5335, 19081, 1012, 102, 0], 
#       [101, 6847, 10665, 1037, 2112, 1997, 2023, 3075, 1998, 3745, 2115, 3463, 102]]
```

## Expected 
```python
# output truncated to max_length (4 as in the example)
# out: [[101, 12040, 1037, 102], 
#       [101, 6847, 10665, 102]]
```

I have updated to version 3.0.2 now and the tokenizer is working properly. I am closing this issue.