Import error when fine-tuning mbart from master branch
Author: thevasudevgupta
Pull request: no

## 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: 3.3.1
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.6.0+cu101 (True)
- Tensorflow version (GPU?): 2.3.0 (True)
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
<!-- Your issue will be replied to more quickly if you can figure out the right person to tag with @
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 Please tag fewer than 3 people.
 
 tokenizers: @mfuntowicz
 Translation: @sshleifer
 Bart: @sshleifer
 -->
@mfuntowicz @sshleifer

## Information

Model I am using (mBART):

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)
My script involves fine-tuning mbart for multilingual-translation. Problem is arising when importing transformers from master branch.

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)
IITB Hindi-english dataset

## To reproduce

Steps to reproduce the behavior:

1. `import transformers` from master branch

```
 from transformers import (
  File "/content/drive/My Drive/hin-eng/transformers/__init__.py", line 68, in <module>
    from .data import (
  File "/content/drive/My Drive/hin-eng/transformers/data/__init__.py", line 6, in <module>
    from .processors import (
  File "/content/drive/My Drive/hin-eng/transformers/data/processors/__init__.py", line 6, in <module>
    from .squad import SquadExample, SquadFeatures, SquadV1Processor, SquadV2Processor, squad_convert_examples_to_features
  File "/content/drive/My Drive/hin-eng/transformers/data/processors/squad.py", line 10, in <module>
    from ...tokenization_bart import BartTokenizer
  File "/content/drive/My Drive/hin-eng/transformers/tokenization_bart.py", line 18, in <module>
    from .tokenization_roberta import RobertaTokenizer, RobertaTokenizerFast
  File "/content/drive/My Drive/hin-eng/transformers/tokenization_roberta.py", line 20, in <module>
    from .tokenization_gpt2 import GPT2Tokenizer, GPT2TokenizerFast
  File "/content/drive/My Drive/hin-eng/transformers/tokenization_gpt2.py", line 27, in <module>
    from .tokenization_utils_fast import PreTrainedTokenizerFast
  File "/content/drive/My Drive/hin-eng/transformers/tokenization_utils_fast.py", line 29, in <module>
    from .convert_slow_tokenizer import convert_slow_tokenizer
  File "/content/drive/My Drive/hin-eng/transformers/convert_slow_tokenizer.py", line 25, in <module>
    from tokenizers.models import BPE, Unigram, WordPiece
ImportError: cannot import name 'Unigram'
```
## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
Importing transformers normally.

Comments:
Could you fill-in the issue template for bugs so that we may help you?

Please checkout now.

you need to upgrade tokenizers.
It should happen for you if you run `pip install -e ".[dev]"` from the root of the repo.

Thanks, it worked.

> Thanks, it worked.

can you tell  me how did you solve it?thanks



> > Thanks, it worked.
> 
> can you tell me how did you solve it?thanks

Just run the command which @sshleifer suggested from the directory having setup.py file. 

Hello,
Which directory are you talking about ? The transformer site package directory, the project directory ?