Repository: transformers
Issue: Inconsistency between the tokenization of `CLIPTokenizer` and `CLIPTokenizerFast` with `openai/clip-vit-base-patch32`
Labels: WIP

## Environment info
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- `transformers` version: 4.8.2
- Platform: Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.10
- PyTorch version (GPU?): 1.9.0+cu102 (False)
- Tensorflow version (GPU?): 2.5.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help
@patil-suraj, I think you worked on CLIP, maybe you could help me by confirming that this behavior is not normal. If it is and no one can deal with it first, I'd be happy to try to fix it.
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Models:

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- rag: @patrickvonplaten, @lhoestq
- tensorflow: @Rocketknight1

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
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## Information

Model I am using (Bert, XLNet ...): CLIP

## To reproduce

The easiest way to reproduce is to open [this google colab](https://colab.research.google.com/drive/1JzlYtuG4MdAKl8lPI5PkqGcYbdM3N24x?usp=sharing)

Steps to reproduce the behavior:

1. Import the slow and fast CLIP tokenizers from the transformers library and eventualy the tokenizer of https://github.com/openai/CLIP
```
from transformers import CLIPTokenizer, CLIPTokenizerFast
tokenizer_slow = CLIPTokenizer.from_pretrained("openai/clip-vit-base-patch32")
tokenizer_fast = CLIPTokenizerFast.from_pretrained("openai/clip-vit-base-patch32")
```
```
from CLIP import clip as clip_orig
```
2. Tokenize the same text with the 3 tokenizers
```
text = "A photo of a cat"
context_length = 77
```
```
tokens_ids_orig = clip_orig.tokenize(text)
tokens_ids_slow = tokenizer_slow.encode(text, padding="max_length", max_length=context_length, return_tensors='pt')
tokens_ids_fast = tokenizer_fast.encode(text, padding="max_length", max_length=context_length, return_tensors='pt')
```
3. Compare the outputs
```
(tokens_ids_orig == tokens_ids_slow).sum() == context_length
```
Output: `True`
```
(tokens_ids_orig == tokens_ids_fast).sum() == context_length
```

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## Expected behavior

I think I would have expected the slow and fast versions to tokenize the text in the same way. 

<!-- A clear and concise description of what you would expect to happen. -->