add new field, and get errors
Hi, 

could you please post this question on our [Forum](https://discuss.huggingface.co/) as we keep issues for bugs and feature requests?  > Hi,
> 
> could you please post this question on our [Forum](https://discuss.huggingface.co/) as we keep issues for bugs and feature requests?

ok.
after adding new field **tokenized_examples["example_id"]**, and get errors below,
I think it is due to changing data to tensor, and **tokenized_examples["example_id"]** is string list 
**all fields**
```
***************** train_dataset 1: Dataset({
    features: ['attention_mask', 'end_positions', 'example_id', 'input_ids', 'start_positions', 'token_type_ids'],
    num_rows: 87714
})
```

**Errors**
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/transformers/tokenization_utils_base.py", line 705, in convert_to_tensors
    tensor = as_tensor(value)
ValueError: too many dimensions 'str'
```