Add progress_fn argument to MultiWorker.next
Summary:
Adds an optional argument progress_fn to `MultiWorker.next`. Every
time `next ()` is called, `progress_fn ~start ~length ~total` will be called,
where `total` is the total number of items, and `next()` just returned `start`
to `start + length`.
Flow is adding the ability to stream its current status, and its useful to be
able to see the current status of a `Multiworker` job.
Reviewed By: samwgoldman
Differential Revision:
D6210087
fbshipit-source-id:
89803ff03cc96e2fc69946920d3882b76bf08136