Thanks for your respond. It’s still used, as you can see, it’s used in "def add_param_group() ", and as a judge part of a sentence. I think defaults is such as lr=0.3, so the name=‘lr’ and default=0.3.
It is so hard for me to understand why default equal required(_RequiredParameter()) can work.
Maybe I should give it up. Just remember, don’t delve into it