Fix DataCollatorForWholeWordMask
# What does this PR do?

This is a quick fix for #8378 

There is another `_tensorize_batch` in other class, I think we can replace it too. 
And there might be tests missing?
@sgugger #8308
Oh I had missed that this one was a subclass of `DataCollatorForLanguageModeling`, thanks for fixing. The last one isn't a subclass, but we can certainly change it to use the same function too!<|||||>Merging the PR to have the fix as quickly as possible, but if you want to add a test of this data collator, please go ahead! That will make the library more robust.