[onert/frontend] Revisit traininfo metadata name

[zetwhite]

This PR adds 'const' keyword to the traininfo metadata name.
It also adds 'extern' keyword to follow one-definition-rule.

ONE-DCO-1.0-Signed-off-by: SeungHui Youn sseung.youn@samsung.com

related issue : #11692

[zetwhite]

While I was on the next step (next PR), I found an error :

ONE/runtime/onert/frontend/circle_traininfo/include/traininfo_loader.h:30:56: 
error: ISO C++ forbids converting a string constant to ‘char*’ [-Werror=write-strings]
   30 | static constexpr char *const TRAININFO_METADATA_NAME = "CIRCLE_TRAINING";
      |                                                        ^~~~~~~~~~~~~~~~~

It looks like constexpr char* doesn't mean that char-array(string) content is constant.
So, For clearness, Let's just remove constexpr and use const instead.

---

I refered stack-overflow https://stackoverflow.com/questions/30561104/const-constexpr-char-vs-constexpr-char.
I tried to find an official C++ document about this, But it's hard to find a clear explanation about this.

[ragmani]

Could you add a comment like below?

// TODO Change to inline variables since C++17

[zetwhite]

updated!

[YongseopKim]

👍

[YongseopKim]

LGTM and I have a question.

by zetwhite

This PR adds 'const' keyword to the traininfo metadata name.
It also adds 'extern' keyword to follow one-definition-rule.

Where can I find the reference to one-definition-rule?

[zetwhite]

Where can I find the reference to one-definition-rule?

@YongseopKim
Ah, I didn't put the reference about one-definition-rule.
I put things below :

• cpp-reference
  Here, You could find an official document about one-definition-rule

• best way to declare and define global variable : link-from-stackoveflow
  Since the official document is a bit broad, I recommend reading this note (I found this on StackOverflow)
  This one is not highly reliable as an official document, but easy to catch this PR's changes.

[jyoungyun]

LGTM

[ragmani]

LGTM