[mpmd] Integrate PipelineSchedule API into jit lowering flow
Lowering Flow:
When a `PipelineSchedule` is provided in `MpmdConfig`, the `mpmd.jit` lowering
process first runs the IMPORT pass only to generate initial fragments. The rule
builders defined within the `PipelineSchedule` are then invoked to generate
scheduling and merging rules based on these fragments. These rules are used
during the subsequent passes, specifically OPTIMIZE. User defined partitioning
options are checked to ensure they don't conflict with options required by
`PipelineSchedule`.
This new API replaces the direct usage of `fragment_merge_rules` and
`fragment_schedule_rules` in `MpmdConfig`.
PiperOrigin-RevId: 869242735
