Fix TFViT
# What does this PR do?

Fix the code example of `TFViTForImageClassification` + fix the integration test.

cc @ydshieh 
LGTM, thanks! 
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I open an issue #14403 for the following.
_________________________________

Hi, I found that there is something wrong (not introduced in this PR).

`model.save_pretrained("tmp", saved_model=True)` will give an error `AttributeError: 'ViTConfig' object has no attribute 'items'`.
(I found this when running `test_saved_model_creation_extended` in TF ViT test script)

Maybe it's better to open a new issue for this.

**Update**
Not specific to `TFViT`, even occurs for `TFBert`
```
from transformers import TFBertModel
model = TFBertModel.from_pretrained("bert-base-uncased")

# failed
for x in model.config.items():
    print(x)

# failed
model.save_pretrained("tmp", saved_model=True)
```
This probably suggests that `test_saved_model_creation_extended` (@tooslow) in `test_modeling_tf_common.py` will fail for all TF models.


Full example:
```
from transformers import TFViTForImageClassification
model = TFViTForImageClassification.from_pretrained('google/vit-base-patch16-224')
model.save_pretrained("tmp", saved_model=True)
```