reduce_util: clamp the apply fn start/end

Summary: In helper apply_over_dim_list, we can clamp the start/end, and skip if we don't need to apply fn to any elements.

Reviewed By: manuelcandales

Differential Revision: D48211547

fbshipit-source-id: 311b271e15baf46fbccaa4bacac072981abe6f61