UsdOutput::IsOutput should check attr validity

[dgovil]

Description of Change(s)

UsdShadeOutput::IsOutput should be checking for attr validity like UsdShadeInput::IsInput does.
Thanks to Maddy Adams for catching this and suggesting the fix.

Fixes Issue(s)

• UsdShadeOutput bool() doesn't check for attr definition #2876

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9105

[FlorianZ]

Hi @dgovil. As a pattern, we try to not do a lot of validity checking in these types of API calls. In this case, for example, we expect the client to supply an already valid UsdAttribute. If we made this change, we think there is a ton of other API that would have to be changed to now follow a similar pattern, or otherwise we run the risk of making things behave inconsistently and muddying the waters in terms of whether or not the call site needs to check for validity. Does that make sense? What are your thoughts?

[FlorianZ]

Wait, I just saw your comment on #2876, and it looks like we are already being inconsistent about what I said we wanted to be consistent about ;-)

Okay, so ignore my previous comment. We'll discuss internally, but it sounds like accepting the PR would make this more consistent actually...

[dgovil]

Edit: oops missed your follow up comment.

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I totally see where you're coming from but I would argue that this change would bring some symmetry and consistency to the API.

IsInput already checks for validity, which in turn means that one can follow the same pattern as prims to pass the Input object from a GetInput call to an if check.

GetOutput would be the outlier here since it doesn't follow its counterpart.

I wouldn't advocate adding the check elsewhere based on this PR, but I do think this would make validation checking on an output feel more consistent with the rest of the USD API.