Add `-Denable-fp16=true` build to CI about nntrainer

[myungjoo]

You may add enable-fp16 build test in CI.
You may limit test coverage or error reporting mechanisms for enable-fp16, too. (you can let github-action gracefully ignore failed TCs, too, so that we can see the number of failed cases but the CI says "OK".)

Anyone needing this may add it. (@heka1024 ?)

[heka1024]

@skykongkong8 @myungjoo I'll add this to CI. (As non-required test) Thanks for comment!

[skykongkong8]

Good point.
As far as I am concerned, Reason why -Denable-fp16 is not included in current CI is because it is relatively newer option than the others.
Even for now, I personally think that it is not really matured to be checked for every PR, because we have multiple plans to refactorize the structure of half-precision formats or related codes.. I need more opinions about this. @jijoongmoon

[taos-ci]

cibot: Thank you for posting issue #2560. The person in charge will reply soon.

[heka1024]

@skykongkong8 What do you think?

[skykongkong8]

Great! Thanks for pointing this issue out :)