[config] Reject unknown pipeline_parallel_schedule names

[YeonwooSung]

Summary

parallelism.pipeline_parallel_schedule accepted any string. A typo such as Interleved1F1B only failed when get_schedule_class ran after the model was built.

Validate through the upstream schedule registry at ParallelismConfig init (same idea as compile.components and context_parallel_load_balancer). Named schedules and CSV class names (PipelineScheduleMulti, etc.) still work. Unknown names fail even when PP is disabled.

Test plan

• pytest tests/unit_tests/cpu/test_parallelism_config.py (13 passed)
• Default 1F1B, Interleaved1F1B, and PipelineScheduleMulti accepted
• foo, Interleved1F1B, and "" rejected