[dask] include multiclass-classification task in tests (#4048)
[dask] include multiclass-classification task in tests (#4048)

* include multiclass-classification task and task_to_model_factory dicts

* define centers coordinates. flatten init_scores within each partition for multiclass-classification

* include issue comment and fix linting error
henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM