[BC-breaking][pipeline] own and pre-warm process-pool executors

[mthrok]

The pipeline now treats a ProcessPoolExecutor passed to PipelineBuilder.pipe(op, executor=...) as a specification rather than a live object to drive. At build time it reads the executor's worker count and initializer and stands up its own equivalent worker pool, which it spawns eagerly and shuts down when the pipeline stops.

This fixes an unsafe pattern: a stdlib ProcessPoolExecutor spawns its worker processes lazily, on demand, as items arrive. In a running pipeline that fork happens mid-load from a process that already has the event-loop thread (and other threads) live, and forking a multi-threaded process can deadlock or corrupt the child. Spawning all workers up front, before the event-loop thread starts, removes that hazard. The in-process build reuses the same eager, fork-safe _hoist_process_pools machinery that run_pipeline_in_subprocess already used, so the two paths now handle process pools consistently. A ProcessPoolExecutor shared across stages maps to a single owned pool, reaped once.

build_pipeline and PipelineBuilder.build gain an mp_context argument so callers can select the multiprocessing start method (e.g. "spawn" or "forkserver") for those spawned workers — the real lever for fork-safety alongside eager pre-warming.

Because the pipeline only reads the executor as a spec, an executor handed over after it has already spawned workers or had work submitted now triggers a RuntimeWarning (its own workers are not used and the caller still owns it) instead of being rejected with ValueError. This warn-not-reject policy is unified across the in-process, run_pipeline_in_subprocess, and stage-fusion paths (_ensure_executor_unused becomes _warn_if_executor_used).

Thread- and interpreter-pool executors are unchanged by this diff; recreating those as pipeline-owned, async-aware workers is a planned follow-up.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D109949075. (Because this pull request was imported automatically, there will not be any future comments.)