Support Train Test Data split in Data Generation

[Yujun-Zou]

In the current user flow, If we only use dataset generation, combination, define our custom function and evaluation we are fine consider all data in dataset generation as test data and input for evaluation. e.g. the headline example.

if we take improvement into consideration, then we cannot use 100% data in dataset generation as data for evaluation and improvement .It is like use both data as trainnig data and testing data.

The suggested modification is to include train test split in the dataset generation. If not specified, it is considered as 100% test data. ow follow the config specification.