augment.mdl_df doesn't work when a mixed model is present

[tanner-ellison]

augment.mdl_df doesn't work when a mixed model is present

tsibbledata::olympic_running %>% 
aggregate_key(Length * Sex, Time = mean(Time)) %>%
model(mean = MEAN(Time), naive = NAIVE(Time)) %>% 
mutate(mixed = (mean + naive) / 2) %>%
augment()

Error : Can't combine `mean` <lst_mdl> and `mixed` <lst_mdl>.
✖ Some attributes are incompatible.
ℹ The author of the class should implement vctrs methods.
ℹ See <https://vctrs.r-lib.org/reference/faq-error-incompatible-attributes.html>.