Investigate possible off-by-one error in EachBatch

The following discussion from gitlab-ce!12584 should be addressed:

• @grzesiek started a discussion: (+2 comments)

  @yorickpeterse It is possible that we still have off by one problem here, because in case of having 3 stages, batch size: 2, range size: 1, we yield

  index: 1, range: [1, 2]
  index: 1, range: [1, 2]
  index: 2, range: [3, 3]

  It will still works, and I guess that is not a big deal, but off-by-one bug might be tricky to debug.