PegasusTokenizer requires the SentencePiece library but it was not found in your environment
Author: marcoabrate
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: 4.0.0
- Platform: Google Colab
- Python version: 3.6.9

### Who can help

 tokenizers: @mfuntowicz
 Pegasus: @patrickvonplaten 

## To reproduce

Steps to reproduce the behavior:
```
!pip install -U transformers
!pip install sentencepiece

from transformers import PegasusTokenizer
tokenizer = PegasusTokenizer.from_pretrained('google/pegasus-large')
```

Error:
```
ImportError                               Traceback (most recent call last)

<ipython-input-7-12d68b5e397b> in <module>()
      1 from transformers import PegasusTokenizer
----> 2 tokenizer =    PegasusTokenizer.from_pretrained('google/pegasus-large')

/usr/local/lib/python3.6/dist-packages/transformers/utils/dummy_sentencepiece_objects.py in from_pretrained(self, *args, **kwargs)
     54     @classmethod
     55     def from_pretrained(self, *args, **kwargs):
---> 56         requires_sentencepiece(self)
     57 
     58 

/usr/local/lib/python3.6/dist-packages/transformers/file_utils.py in requires_sentencepiece(obj)
    459     name = obj.__name__ if hasattr(obj, "__name__") else obj.__class__.__name__
    460     if not is_sentencepiece_available():
--> 461         raise ImportError(SENTENCEPIECE_IMPORT_ERROR.format(name))
    462 
    463 

ImportError: 
PegasusTokenizer requires the SentencePiece library but it was not found in your environment. Checkout the instructions on the
installation page of its repo: https://github.com/google/sentencepiece#installation and follow the ones
that match your environment.
```


Comments:
Hey @marcoabrate, 
I tried to reproduce your error in this colab without success: https://colab.research.google.com/drive/1nBCEtP773LplNodOSw5OBW-rJ84gizW5?usp=sharing can you check again ?

You are right @patrickvonplaten 
To reproduce

```
!pip install -U transformers

from transformers import PegasusTokenizer
tokenizer = PegasusTokenizer.from_pretrained('google/pegasus-large')

!pip install sentencepiece

tokenizer = PegasusTokenizer.from_pretrained('google/pegasus-large')
```

Very weird

Having the same issue. Although solely in my Jupyter notebook. Running the code from a file is working fine..

Probably it's a way the Jupyter kernel works. Indeed, if you restart the kernel and install `sentencepiece` before is working.

as @marcoabrate said, restated kernel at my project and without code changes everything started working

I can confirm that this solution is still valid today. I encountered the same issue today.
I solved it by adding ` !pip install sentencepiece` and then fully restart the Jupiter environment and rerun.

I am unable to install  the SentencePiece library, this is the error i get when i do pip3 install sentencepiece:

error: legacy-install-failure


Restarting the kernel and using
!pip install Transformers==3.2.0 instead of !pip install Transformers, worked for me

After using !pip install sentencepiece just  restart the kernel and run the cells. It will work fine