Separate communication groups for different stages

[jsjason]

We currently use a separate communication group for each stage. However, the user can't set the number of Tasks differently for each stage, which makes multiple communication groups pretty meaningless. What do you think, @kijungshin?

[kijungs]

@jsjason It is difficult to reuse a communication group among stages because topologies and datatypes of communication are usually different.
What do you mean is simply using one communication group regardless of reusage?

[jsjason]

The user doesn't use the communication group names anywhere other than when defining UserJobInfo.getStageInfoList(). Furthermore, the user would not care of the class name at all since the user doesn't need it in the algorithm itself. Thus I think it's better for Dolphin to hide the group names from the user.

[jsjason]

I understand it's hard to actually implement a Dolphin core that hides the static group names, since the user can use an unbounded number of stages. Nonetheless, we shouldn't throw the group name definition burden to the user unless the user really needs to use those names.