[tuner] Candidate specs should be annonated with default unit attr

[bangtianliu]

All generated candidate specifications should be annotated with iree_codegen.tuning_spec_with_default_entrypoint during the tuning phase. This annotation indicates that the generated tuning specification must include a named sequence operation marked with __kernel_config.

The annotated attribute can be used later, for example, to merge multiple tuner-genertaed spec into one tuning spec file.