[Fix doc example] FSMT
# What does this PR do?

Fix doc example. 

Without this fix, I get the following error


```
            batch_size = inputs_tensor.shape[0]
          File "/home/yih_dar_huggingface_co/transformers/src/transformers/tokenization_utils_base.py", line 253, in __getattr__
            raise AttributeError
        AttributeError
```

where `inputs_tensor` has type `BatchEncoding`.
_The documentation is not available anymore as the PR was closed or merged._