Continue training args and tqdm in notebooks
Added a little more description to the metadata of training args for new scripts. 
`--overwrite_output_dir` can be used to continue training from checkpoint if `--output_dir` points to a directory already having checkpoints.

This can help reduce the confusion when migrating from older script.


tqdm in colab prints every step in new line.
so updated to 
`from tqdm.auto import tqdm`
Updates committed<|||||>Thanks!