Auto-resume training from checkpoint
# What does this PR do?

Feature suggested by @madlag. 
In the examples scripts, change the current behavior so if checkpoints are present in the output_dir passed to the script command, the training resumes from there. If the `output_dir` exists, is nonempty but has no checkpoint, the same behavior as before is applied (error). If it exists with checkpoints inside, the last checkpoint is grabbed to resume training from there. If `--overwrite_output_dir` is passed, the folder is destroyed as before.

This avoids user having to pass `output_dir/checkpoint-xxx` as their model name or path to resume training from a checkpoint, which is a nice improvement. The bad surprise can be if you set that `output_dir` with a trained model you like by mistake, but at the same time, the training is resumed from the last checkpoint so shouldn't be too long (and will converge to the same model) and interrupting before the end will not erase the model inside the folder, so I think the pros outweigh the cons.

Tweak the `run_glue` example script for now, will expand it to all scripts if accepted.
I was having a problem getting this to actually resume training on my system, and I had to make three small changes to the new code in trainer_utils.py:

1. `checkpoints = [path for path in content if _re_checkpoint.search(path) is not None and os.path.isdir(path)]` was returning empty. I changed `os.path.isdir(path)` to `os.path.isdir(os.path.join(folder, path))` and now it returns a list of the checkpoint folders as expected.
2. Similarly, the `get_last_checkpoint` function was returning the basename of the checkpoint folder, not the full path, which seems to be expected based on the updates to the example scripts. I changed the last line of the function to `return os.path.join(folder, max(checkpoints, key=lambda x: int(_re_checkpoint.search(x).groups()[0])))`
3. After I made those update, it was resuming from the oldest checkpoint, not the newest. I noticed the checkpoint regex was only capturing the final digit in the directory name. I changed it to `_re_checkpoint = re.compile(r"^" + PREFIX_CHECKPOINT_DIR + r"\-(\d+)$")` with the `+` inside the capture group, and now `get_last_checkpoint` is giving me the newest checkpoint as expected.

I'm just a novice, so I'm not sure if those tweaks would break anything other systems. Does `os.listdir()` return full paths instead of basenames under some OS/python combinations?

But with these changes I'm able to resume an aborted training from within the same folder. <|||||>Oh, I went a bit too fast and all the issues you list are completely valid! You should make a PR with all your changes since you were the one to find the problems and fix them :-)<|||||>Got sucked back into work for my day job, but I'll try getting to that soonish.

Unrelated, do you guys have an office in DUMBO? I think we're in the same building (at least until we all started working from home)<|||||>DUMBO offices are at 20 Jay street, you should come and say hi for a coffee when the pandemic is over if you're around :-) <|||||>Yeah, I run a small animation studio up on the 10th floor. Turns out the fancy GPUs I have for animation are also good for messing around with machine learning :) And so my pandemic side project became teaching my computer to write poetry. 

Someday when it's safe again I'll definitely stop by.<|||||>Haha small world @jncasey! I'm missing 20 Jay right now. What is your company's name? <|||||>We're called Mixtape Club. We moved into the building from Manhattan last January, and were only there for about 10 weeks before everyone started working from home. Now my business partner is the only one who goes in, since he can't bring our whole sound studio back to his apartment. 