[BNB] fix nasty `bnb` bug
# What does this PR do?

This PR fixes a very nasty bug that can be reproduced with the following script: 
```
from transformers import AutoModel

model_16bit = AutoModel.from_pretrained("bigscience/bloom-560m", device_map="auto", load_in_8bit=False)
model_8bit = AutoModel.from_pretrained("bigscience/bloom-560m", device_map="auto", load_in_8bit=True)

print(model_16bit.is_loaded_in_8bit)
>>> True
```
In fact we should assign the attribute `is_loaded_in_8bit` to the variable `model` instead of `cls`. Otherwise `is_loaded_in_8bit` will be overriden by the next model that is loaded in 8bit

@sgugger @ydshieh 
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