Design a multi-stage programming model

[kijungs]

Objective:

• Design a programming model that is simple but able to express various ML algorithms

Specification:

• Each Job consists of one or more Stages
• Stages are executed on the same evaluators, which maintain Contexts, stage by stage
• Data can be passed among Stages using Key-value Store Service
• Each Stage follows a BSP programming model
• Each Stage consists of one Controller Task and one or more homogeneous Compute Tasks
• The Controller Tasks and Compute Tasks communicate each other through Group Communication
• Group Communication includes BroadCast, Reduce, Gatter, and Scatter of arbitrary types of data
• Each Compute Task and Controller Task consists of initialize, run, and cleanup step.

[jsjason]

Closed via #19.