TF text classification examples
# What does this PR do?

This PR updates the text classification examples to use the more modern `to_tf_dataset()` method, as opposed to a custom function. A few minor fixes were added as I navigated related files.

Example of running command: `python run_glue.py --model_name_or_path distilbert-base-cased --task_name mnli --do_train --do_eval --output_dir $HOME/test_model`
_The documentation is not available anymore as the PR was closed or merged._