permit reduce input types to differ from reduce output types

When map tasks write intermediate data out, they always use SequencialFile RecordWriter with key/value classes from the job object.

When the reducers write the final results out, its output format is obtained from the job object. By default, it is TextOutputFormat, and no conflicts.
However, if one wants to use SequencialFileFormat for the final results, then the key/value classes are also obtained from the job object, the same as the map tasks' output. Now we have a problem. It is impossible for the map outputs and reducer outputs use different key/value classes, if one wants the reducers generate outputs in SequentialFileFormat.

A simple fix would be to add another two attributes to JobConf class: mapOutputLeyClass and mapOutputValueClass. That allows the user to have different key/value classes for the intermediate and final outputs.