Fix QA examples for roberta tokenizer
# What does this PR do?

https://github.com/huggingface/datasets/pull/2586 has changed the SQUAD dataset and doesn't clean anymore the whitespace in questions. This in turn makes the tokenization fail for tokenizers that don't remove whitespace (like RoBERTa): some questions begin with loooots of spaces and so the truncation strategy then fails because the question itself is longer than the max length (the infuriating example number 107709 of the training set for instance).

For more context, see #12880

This PR addresses that by removing the whitespace on the left of questions.