It turns out that the backend is rarely needed when lowering, e.g.,
for lowering callbacks. Whenever we need the backend for lowering,
we must be in single-platform lowering mode (`len(platforms) == 1`)
and we can look up the backend from `platforms[0]`.
However, in some rare cases we can have a custom `XlaBackend` whose
platform matches `platforms[0]`. We rename `backend_or_name` to just `backend`
and we restrict its type to be an optional `XlaBackend` (not a platform
string).
PiperOrigin-RevId: 712926140
fdb6af82d2