code: public Path getDefaultWorkFile(TaskAttemptContext context,
                                   String extension) throws IOException{
      FileOutputCommitter committer = 
        (FileOutputCommitter) getOutputCommitter(context);
      return new Path(committer.getWorkPath(), getUniqueFile(context, "part", 
                                                             extension));
    }
docs: Get the default path and filename for the output format.
  @param context the task context
  @param extension an extension to add to the filename
  @return a full path $output/_temporary/$taskid/part-[mr]-$id
  @throws IOException