:: DeveloperApi ::
Contextual information about a task which can be read or mutated during execution.
Add a callback function to be executed on task completion.
Add a callback function to be executed on task completion. An example use
is for HadoopRDD to register a callback to close the input stream.
Will be called in any situation - success, failure, or cancellation.
(Since version 0.8.1) use partitionId