Better warning when loading a tokenizer with AutoTokenizer w/o Sneten…
…cePiece

Currently, initializing a `sentencepiece` `AutoTokenizer` without having `sentencepiece` installed results in the following error:

```
AttributeError: 'NoneType' object has no attribute 'from_pretrained'
```
This improves the error message to:
```
 This tokenizer cannot be instantiated. Please make sure you have `sentencepiece` installed in order to use this tokenizer.
```

Fix #8864
Thanks!