ValueError: `steps_per_epoch=None` is only valid for a generator based on the `keras.utils.Sequence`
keras 自己写了个generator，然后继承的keras.utils.Sequence，但是在fit_generator()的时候，会报如下错误：ValueError: `steps_per_epoch=None` is only valid for a generator based on the `keras.utils.Sequence`就会很懵逼，明明继承了sequence类，然后isinstance(your_generator,keras.utils.Sequence)
