

TITLE
Moved feature generation into getitem to save ram

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# What does this PR do?
I had this issue that my Colab Notebook would run out of memory when creating the feature in the SQuAD dataset, so I basically just moved it to the getitem section. I also added a function to force create the features immediatly incase somehow for some reason would want the features created in the beginning

Fixes # (issue)


## Before submitting
- I submitted an issue, but never got any feedback.
- I did not update the documentation due to the fact that it doesn't exist
- I did not write tests either since I couldn't get the tests to work out on my machine. However I did test it while training the model. However I could easily write tests if you believe it's necessary. 


## Who can review?
@patrickvonplaten, maybe you can take a look at the pull request?
