Add AverageModelCheckpoint

[Squadrick]

Closes #903

[wuliytTaotao]

A question about AverageModelCheckpoint, when saving the moving average weights, we should not change the model.trainable_weights for we may keep training later. Does AverageModelCheckpoint change the model's weights after saving function?

[Squadrick]

A question about AverageModelCheckpoint, when saving the moving average weights, we should not change the model.trainable_weights for we may keep training later. Does AverageModelCheckpoint change the model's weights after saving function?

@wuliytTaotao I'll use a parameter that allows both options:

1. Assign the moving average weights to the model, and save them.
2. Keep the old non-averaged weights, but the saved model uses the average weights.

That will cover both scenarios of the user.

[seanpmorgan]

@Squadrick Thanks!! LGTM. Would you mind merging master into this. I believe we'll need to add type checking, but want to verify the CI

[Squadrick]

@seanpmorgan Added type info, and it should be good to merge.

[seanpmorgan]

LGTM Thanks again!

resolved