Incorrect variable name `tokenized_dataset` in Chapter 7 section 6 of course

[jpodivin]

Second portion of the chapter, which focuses on accelerate library, uses tokenized_dataset variable.
However this variable doesn't exist, leading to error if person following the guide attempts to execute the cell.

It is most likely that the variable tokenized_datasets was intended instead.

https://github.com/huggingface/notebooks/blob/main/course/en/chapter7/section6_pt.ipynb

[jpodivin]

Closing in favor of huggingface/course#702 in course repo.