Fix import of meta graphs with partitioned variables into a scope.

Saver inspects SliceInfo to decide the variable name when creating a
checkpoint. Before this fix even if a partitioned variable ("weights")
was imported into a scope "a" it would still be checkpointed as ("weights")
instead of ("a/weights") since import_scoped_meta_graph was not adjusting
the SliceInfo.

WARNING: if you use import_meta_graph on graphs with partitioned_variables WITH an import_scope argument AND then create a Saver to write/read checkpoints this change
may break your checkpoint loading.
PiperOrigin-RevId: 173105796