Simplify column_names in run_clm/mlm
Following https://github.com/huggingface/transformers/pull/21343

Just a minor change to simplify the code, and fix a small bug (`column_names` needs to be a list to be able to call `column_names[0]`)

cc @stas00 
_The documentation is not available anymore as the PR was closed or merged._<|||||>This is much better - thank you, @lhoestq!