add --validation_split_percentage for custom dataset
Author: kornosk
Pull request: yes

# What does this PR do?
In the current version of the example, the `--validation_split_percentage` only works for datasets loaded from the hub but not for custom datasets. If `--do_eval` is set for custom dataset, it requires `--validation_file`.

This PR makes `--validation_split_percentage` works for a custom dataset if `--validation_file` is not set.
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