[Relay] Fix handling a tuple node in op fusion

[masahi]

Fixes #2431. In #2187, I was making a false assumption that the number of fields of a tuple nodes is the same as the number of parameters to a function containing the tuple. But this is not always the case, since the field value can be generated inside the function (or group).

For example, the gluon SSD model returns class ids, bounding box scores, and locations, shown below. After converting to Relay, the three outputs are put into a tuple. Three slice_axis and the last tuple are put into the same group. This group has one parameter, result, but the tuple has three fields. This caused out of bounds access in #2431.

  result = ...
  ids = F.slice_axis(result, axis=2, begin=0, end=1)
  scores = F.slice_axis(result, axis=2, begin=1, end=2)
  bboxes = F.slice_axis(result, axis=2, begin=2, end=6)
  return ids, scores, bboxes

@tqchen @kevinthesun @jroesch please review.

[tqchen]

Thanks @masahi , this is now merged