LongformerTokenizer Error
## 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: 2.11.0
- Platform: Linux-5.10.68+-x86_64-with-debian-buster-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.5.1 (True)
- Tensorflow version (GPU?): 2.2.0 (True)
- Using GPU in script?: fill in
- Using distributed or parallel set-up in script?: fill in

### Who can help
@patrickvonplaten

## Information

Model I am using (Bert, XLNet ...):
Longformer
The problem arises when using:
* [ ] the official example scripts: (give details below)
>>> from transformers import LongformerTokenizer, LongformerForSequenceClassification
>>> import torch

>>> tokenizer = LongformerTokenizer.from_pretrained('allenai/longformer-base-4096')
>>> model = LongformerForSequenceClassification.from_pretrained('allenai/longformer-base-4096')

>>> inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-509e5550f58e> in <module>
----> 1 inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")

TypeError: 'LongformerTokenizer' object is not callable
