[pipeline] Add batched routing to path_variants

[mthrok]

Add a batched option to PathVariants and PipelineBuilder.path_variants.

When set, the router receives a whole batch (a list) and returns one path index per element; the batch is partitioned into per-path sub-batches (order preserved), each path processes its sub-batch as a list, and the fan-in merge concatenates the sub-batches back into one batch.

Per-item routing pays the router + fan-out/fan-in machinery once per element. For workloads that route in bulk -- e.g. a cache hit/miss split over an already-aggregated batch -- that fixed per-item cost dominates. Batched routing amortizes it over the whole batch: the router runs once per batch, each branch runs its ops once per batch, and one list (not one item) crosses each queue per batch.

Semantics:

• Each input batch contributes exactly one sub-batch (possibly empty) to every path queue, so the queues stay in lockstep and the merge recombines an input batch's sub-batches by reading one list from each path.
• A branch may drop elements by returning a shorter list, and must tolerate an empty input list (a path that received no elements for a batch).
• A batched router must return exactly one index per element; otherwise the stage fails.

batched defaults to False, so existing per-item behavior is byte-identical (not backward-incompatible).

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D112745035. (Because this pull request was imported automatically, there will not be any future comments.)