Chain returned by `predict` will contain variables used in `:=` statements

[sunxd3]

Note: this is only effective after #716 is merged.

This is because we are using values_as_in_model to extract values in predict. ref code

[penelopeysm]

@sunxd3 are you planning to take this on? If not I can do it :)

[sunxd3]

I probably won't have time to do this, thanks Penny!