I just discussed on Twitter with Nick Doiron that published the notebook ByT5-Finetuning-Datasets.ipynb.
He wrote the following tweet on the issue showed in my notebook:

It must be due to recent changes - transformers==4.9.2 works.  decode got broken
The issue is because instead of 256 byte tokens, there are 3 special bytes (total token ids 0-258) and it isn’t accounting for those

My notebook has been updated with the following code and now, it works 
!pip install transformers==4.9.2
I guess that the HF team will solve this issue in the next transformers version.