fix is casual for qwen3

[leizhenyuan]

This issue is the same for #2868
As we found when training qwen3, is_casual is not properly set.

[Datta0]

That's a nice observation. Probably not identified thus far cuz FA and xformers take precedence. This only happens in the niche case where both those don't exist and we don't pass mask

[Datta0]

But this might need to be changed across the entire spectrum
Like mistral, gemma etc as well