Task cbind breaks when task's backend has primary_key different to `..row_id`

[sebffischer]

library(mlr3verse)
#> Loading required package: mlr3
library(data.table)

d = data.table(
  x = factor(letters[1:10]),
  y = rnorm(10),
  my_key = 1:10
)

backend = as_data_backend(d, primary_key = "my_key")

task = as_task_regr(backend, target = "y")

learner = as_learner(ppl("robustify") %>>% lrn("regr.rpart"))

learner$train(task)
#> Error: All backends to rbind must have the primary_key 'my_key'
#> This happened PipeOp encode's $train()

^{Created on 2023-08-31 with reprex v2.0.2}

[mb706]

probably an issue with Task$cbind()

[sebffischer]

When a data.frame is passed to Task$cbind as_data_backend.data.frame is called which automtically sets the primary key to ..row_id

[sebffischer]

we could handle both cases:

1. A data.frame is passed to $cind() --> then we create the primary_key under the name of the existing primary_key
2. A backend is passed to $cbind() --> then we can call DataBackendRename in case the primary key's don't match and the primary key of the task's backend is not a column name in the backend passed to $cbind().