TypeError: cannot create 'BPE' instances
# 🐛 Bug

## Information

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

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

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

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:
01-training-tokenizers.ipynb
```
# For the user's convenience `tokenizers` provides some very high-level classes encapsulating
# the overall pipeline for various well-known tokenization algorithm. 
# Everything described below can be replaced by the ByteLevelBPETokenizer class. 

from tokenizers import Tokenizer
from tokenizers.decoders import ByteLevel as ByteLevelDecoder
from tokenizers.models import BPE
from tokenizers.normalizers import Lowercase, NFKC, Sequence
from tokenizers.pre_tokenizers import ByteLevel

# First we create an empty Byte-Pair Encoding model (i.e. not trained model)
tokenizer = Tokenizer(BPE())

# Then we enable lower-casing and unicode-normalization
# The Sequence normalizer allows us to combine multiple Normalizer that will be
# executed in order.
tokenizer.normalizer = Sequence([
    NFKC(),
    Lowercase()
])

# Our tokenizer also needs a pre-tokenizer responsible for converting the input to a ByteLevel representation.
tokenizer.pre_tokenizer = ByteLevel()

# And finally, let's plug a decoder so we can recover from a tokenized input to the original one
tokenizer.decoder = ByteLevelDecoder()
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
Flawless Run
## Environment info
- `transformers` version: 2.10.0
- Platform: Linux-5.4.0-31-generic-x86_64-with-debian-bullseye-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.4.0 (False)
- Tensorflow version (GPU?): 2.1.0 (False)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

Specific recurring Error:
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-f099004b011b> in <module>
     10 
     11 # First we create an empty Byte-Pair Encoding model (i.e. not trained model)
---> 12 tokenizer = Tokenizer(BPE())
     13 
     14 # Then we enable lower-casing and unicode-normalization

TypeError: cannot create 'BPE' instances

NB. I have gone through this issue:https://github.com/huggingface/transformers/issues/3787,
It dosent solve it either.

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.
