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ValueError: Tokenizer class T5Tokenizer does not exist or is not currently imported.

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 @mfuntowicz

## 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: Latest  transformers==4.2.0.dev0
- Platform:  Colab
- Python version:  Python 3.6.9
- PyTorch version (GPU?): torch==1.7.0+cu101
- Tensorflow version (GPU?):
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@mfuntowicz
## Information



The following code indicated in the latest HF news letter seems to have isssues when I tried
I get tokenizer error both under Fast and Slow (True/Flase tokenizer parameter) conditions when I had checked


The problem arises when using:
* [ ] the official example scripts: (give details below)
* [ ] 
```python
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM 

tokenizer = AutoTokenizer.from_pretrained("mrm8488/mT5-small-finetuned-tydiqa-for-xqa",use_fast=False )

model = AutoModelForSeq2SeqLM.from_pretrained("mrm8488/mT5-small-finetuned-tydiqa-for-xqa")

context = "HuggingFace won the best Demo paper at EMNLP2020."
question = "What won HuggingFace?"
input_text = 'question: %s context: %s' % (question, context)
features = tokenizer([input_text], return_tensors='pt')
output = model.generate(**features)
tokenizer.decode(output[0])
```

## To reproduce

Steps to reproduce the behavior:

1. Run the above code on Google Colab

<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     -->
**ERROR reported**

`ValueError                                Traceback (most recent call last)
<ipython-input-3-87256159791c> in <module>()
     10 from transformers import AutoTokenizer, AutoModelForSeq2SeqLM
     11 
---> 12 tokenizer = AutoTokenizer.from_pretrained("mrm8488/mT5-small-finetuned-tydiqa-for-xqa",use_fast=False )
     13 
     14 model = AutoModelForSeq2SeqLM.from_pretrained("mrm8488/mT5-small-finetuned-tydiqa-for-xqa")

/usr/local/lib/python3.6/dist-packages/transformers/models/auto/tokenization_auto.py in from_pretrained(cls, pretrained_model_name_or_path, *inputs, **kwargs)
    358             if tokenizer_class is None:
    359                 raise ValueError(
--> 360                     "Tokenizer class {} does not exist or is not currently imported.".format(tokenizer_class_candidate)
    361                 )
    362             return tokenizer_class.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)

ValueError: Tokenizer class T5Tokenizer does not exist or is not currently imported.`


