[dask] run one training task on each worker (#4132)
[dask] run one training task on each worker (#4132)

* [dask] run one training task on each worker

* add comment on pure

* missing ticks

* empty commit
microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM