[BE] Use `value_or` (#103065)
`s/!k.has_value || *k == foo/k.value_or(foo) == foo/`
Which yields the same code see https://godbolt.org/z/6b35zYcYc
<!--
copilot:summary
-->
### <samp>🤖 Generated by Copilot at 003c703</samp>
Simplify the logic for registering and looking up backend fallbacks in `library.cpp` by using a constant and an optional helper.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103065
Approved by: https://github.com/albanD, https://github.com/Skylion007
