[python] disabled early stopping in dart mode

[StrikerRUS]

Refer to #1893 (comment).

Maybe error is better, because users tend to completely ignore warnings?

[guolinke]

@StrikerRUS I think we can disable early_stoping when using dart, even when early_stopping is set.

[StrikerRUS]

@guolinke At cpp side?

[guolinke]

@StrikerRUS the change of cpp side is in another PR.
The logic of early stopping is independent in cpp/python/R, so we need to disable it in python side as well.

[StrikerRUS]

@guolinke Ah, I see!

[StrikerRUS]

@guolinke I've updated PR. Unfortunately, I'm not familiar with callback module yet, therefore this solution might be ugly. Please check.

[guolinke]

maybe this can be moved into _init(env) ?

[StrikerRUS]

@guolinke Sorry, I was in a hurry and mislooked this aspect, which made me think that it's impossible to set enabled variable in _init function. I've reworked this.

[Teeeto]

Sometimes early stopping is needed, disabling it may render dart mode quite useless. I'd suggest to save/override the best model (or at least a mask for dropped trees) in memory at each iteration and return the proper model when the early stopping is triggered.