[python-package] add type hints on Booster.set_network() (#4068)
[python-package] add type hints on Booster.set_network() (#4068)

* [python-package] add type hints on Booster.set_network()

* change behavior
henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM