User-friendly exception when LightningWork.run() method is missing

[awaelchli]

What does this PR do?

Fixes #14677

The LightningWork no longer is an abstract class, and the run() method is no longer abstract.
When you try to instantiate a work without a run method overridden, you will get an error message saying:

The work name of the class is missing the run() method. This is required. Implement it first and then call it in your Flow.

Does your PR introduce any breaking changes? If yes, please list them.

No.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda

[codecov]

Codecov Report

Merging #14759 (c0bc164) into master (8f14184) will increase coverage by 1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #14759     +/-   ##
=========================================
+ Coverage      84%      85%     +1%     
=========================================
  Files         395      328     -67     
  Lines       28817    25784   -3033     
=========================================
- Hits        24105    21871   -2234     
+ Misses       4712     3913    -799     

[tchaton]

LGTM !

[krshrimali]

Thank you @awaelchli - I'm a bit late to the party, sorry for that. I left a comment but great work on getting this PR in, let's get it in the next release. 🚀

I changed the milestone to app:0.6.x, is that okay? Please feel free to move it back to 0.7 if desired.

[krshrimali]

Thank you so much @awaelchli for picking this up.

I really liked the message here: https://github.com/Lightning-AI/lightning/pull/14760/files#diff-7fc1f959c169d77f9190069631cd8e51de55be8cc19214c92440cee6cb42f171R129-R130, with a minor "Please", and also "work" -> "Work" (just like Flow is used instead of flow). I know this is very minor, but this is just one of those errors that is going to reach to the user a lot of times.

Probably:

f"The Work `{self.__class__.__name__}` is missing the `run()` method. This is required. Please implement it"
                " first and then call it in your Flow. You can refer to the docs here: https://lightning.ai/lightning-docs/basics.html?highlight=lightningwork on some basics around `LightningWork`"

(I'm not sure if including the docs link is a good idea here, but just pointing them to the place where they can learn how to implement the run method will help)

Nothing major, and will have to be done in another PR, but I'm just nit-picking here. (please don't mind) ❤️

cc: @Felonious-Spellfire on his opinion as well! I'm not a native English speaker, so I'm sorry if I'm wrong :)

[awaelchli]

Feel free to open a PR with your ideas. Adding the doc link to the message is risky business. We haven't considered that so far as there is an open problem of how to guarantee valid links over time, so not sure if we are ready to do that just yet.