Moved feature generation into getitem to save ram
# 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?

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
