Addition of on-the-fly loading for MLM training and fix for default pad_to_max_length value for TPU
Author: DarshanDeshpande
Pull request: yes

# What does this PR do?

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Fixes #10204, #10024


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## Who can review?
@sgugger @lhoestq @patil-suraj

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Models:

- albert, bert, xlm: @LysandreJik
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Comments:
Thanks for your PR! We don't want to switch the examples to use on-the-fly tokenization however as in most cases it's actually faster to do it once and for all. Having to do it on-the-fly for a training with huge data is more of a specific use-case. Your PR can be referenced as an example of how to do it in practice but I don't think we will merge it.