enhancement/save_model_each_k_epochs

[troeshust]

I use an external custom script to check the model quality and compute the metrics. In order to monitor overfitting and to perform an early stopping, I need to run this script not only on the final best-model, but also on the models resulting from the intermediate epochs. There is already a save_model_at_each_epoch parameter (this line) in the Flair, but due to the large size of the model, there is not enough disc space to save each epoch.
It would be nice to enhance this parameter and add the ability to save the model every k-th epoch.

[stale]

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