Add [[maybe_unused]] attribute to TfPyObjWrapperStub._stub

[dgovil]

I'm trying to reduce a bunch of the warnings that take up the build logs (I build often...so any reduction really helps). TfPyObjWrapper is one of the heavy offenders in the logs, but _stub is necessary per #1716 , so this just says that _stub can be unused.

NOTE: This does require C++17, I'm just putting it up now so that it's logged for whenever that happens.

I also think this is something that should be applied to _schemaTokens in the API schemas too, as those are the other major offenders. But I can hit that up in a different PR if this one doesn't raise any concerns.

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7925

[nvmkuruc]

Now that OpenUSD is on C++17, @dgovil do you think it's a good time to consider integrating this PR?

[dgovil]

Yep that would be great if we could. I just rebased on dev to resolve the conflicts that had accumulated.
I did also add -Wno-ignored-attributes for gcc, because unfortunately the situation is:

1. MSVC and Clang warn for unused attributes, so this attribute quells them.
2. GCC doesn't warn for it but then warns that the attribute does nothing as a result.

[nvmkuruc]

That's a great catch for GCC. If there's architecture dependent behavior, would it be better to create a define in pxr/base/arch (ie. ARCH_MAYBE_UNUSED_PRIVATE_MEMBER) and set it to [[maybe_unused]] in the MSVC and Clang code paths and let it be empty for GCC?

[dgovil]

Ah you know what, I missed after rebasing that Pixar have already added ARCH_PRAGMA_UNUSED_PRIVATE_FIELD above that line. So I'll just close out this PR as unnecessary.

[nvmkuruc]

👍