Refactor pipeline_parallel.py for graph PP reuse

Extract get_pipeline_metadata, split_module, and
get_pp_rank_to_stage_indices_mapping as standalone functions from the
monolithic pipeline_llm. Add backward_requires_autograd and scale_grads
params to build_pipeline_schedule. No behavior change for existing PP.

stack-info: PR: #2724, branch: sanketpurandare/stack/3

Author: sanketpurandare