Additional attributes for Benchmark

[zhu0619]

Description

To facilitate for both UI searching and downstream usage (such as identify the metrics for the corresponding scenario),
I suggest adding attributes for Benchmark
task_type: reg or cls
num_classes: for multiclass tasks , default binary

Acceptance Criteria

• Attributes have been added.

[cwognum]

I will do a pass soon! Also want to add the number of datapoints for the dataset for example.