CER Normalization

I'm not finding in the documentation a method to normalize CER metric. Do you have this implemented?

```
cer.features
OUTPUT:
{'predictions': Value(dtype='string', id='sequence'),
'references': Value(dtype='string', id='sequence')}

df[df['cer'] == 1.25]
OUTPUT:
GT | OCR_Prediction | wer | cer
2.49 | 749.00 | 1.0 | 1.25
2.49 | 749.00 | 1.0 | 1.25
2.50 | 1950.00 | 1.0 | 1.25
```