Assert valid base source for derivative sources (#91711)
We should not allow creating a derived source (e.g. AttrSource), without a valid base source.
It's more reliable to check this in the source `__init__` or `__post_init__` than asserting we have a valid source before passing that to an AttrSource() call.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91711
Approved by: https://github.com/voznesenskym
