Add flash

[amorehead]

• Adds lightning-flash to poe tasks.
• NOTE: We also need to update the README to describe that a manual installation of lightning-flash is in order, via either poe install-flash or pip install pytorch-lightning==1.9.5 && pip install lightning-flash --no-dependencies.

[amorehead]

@a-r-j, IIRC I originally proposed to install lightning-flash using pip install ... --no-dependencies instead of by listing it as a poetry dependency because it was raising errors when I tried to install it while including its dependencies. There seem to be some upstream conflicts between lightning 2.0 and lightning-flash that have yet to be addressed. It's possible these conflicts have been addressed with lightning 2.1.2+ and/or newer versions of lightning-flash. Feel free to try them out to see if they can be included in the poetry dependencies more easily.