ModelCheckpoint need monitor average loss

[Johnson-yue]

Description & Motivation

I know ModelCheckpoint can monitor like "train_loss" , "val_loss" , when the value is min and "every_n_train_steps" is true. but I want to cache "train_loss" or "val_loss" from "every_n_train_steps" to next "every_n_train_steps" ,when cache loss is lower than before, then save model.
Example:
every_n_train_steps = 50, monitor = "train_loss"

from step=50 to step=100, if average "train_loss" is lower than step=[0,50] then save model

Pitch

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Alternatives

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Additional context

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cc @lantiga @Borda