[fix requested] Implement a check to prevent `output_dir` from being a folder within `checkpoint_dir`

[subramen]

I set up my outputdir to be a folder within checkpoint dir because i wanted them all to be in the same folder.

# config.yaml
output_dir: "/a/b/c"
checkpoint_dir: "/a/b"

What I didn't realize is that under the hood tune is copying folders from ckpt_dir into output_dir... including output_dir itself. This ends up in a nested Matryoshka like:

A simple check / assert preventing users from defining their dirs in this way could perhaps prevent this confusion!

cc @pbontrager

[felipemello1]

Hey @subramen , thanks for raising it! Closing this issue since it was fixed in #2181