Update files in transformer_doc as per transformer_doc/en files.

[daredevil3435]

What does this PR do?

Updates the code of preprocessing.ipynb, training.ipynb, quicktour.ipynb as per the corresponding updated notebooks in en directory.

Fixes # (issue) #494

Who can review?

@whitesscott @patrickvonplaten @patil-suraj

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[daredevil3435]

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