Size of task prototype

[sebffischer]

When creating a graph learner that has as input a task with tf columns, the data_prototype that is saved in the learner's state after training contains the arg and value vectors as well as the evaluator and other metadata defined in tf.
This unnecessarily blows up the size of learner states in a way that was not intended.

I think this should be fixed in tf, i.e. 0-lentgh tf vectors should drop discardable metadata.

[sebffischer]

With recent PRs in mlr3 and mlr3misc, this problem should be mostly mitigated:

• fix: remove task prototype when resample mlr3#981
• feat: warn when installing packages with srcrefs mlr3misc#97

[sebffischer]

As we now decided to not merge the warning, we should do something about this.
E.g. in mlr3 when creating the data prototype during $train(), it should be possible to add a function that leanifies each column, e.g. stored in mlr_reflections$data_leanifier$tf. This function would then remove the srcref attribute from the functional columns to avoid overly large object sizes when installing with sourcerefs. In the resample() case this is no problem because the prototype is not kept in the learner state.

[sebffischer]

• Merge generic strip_srcref to mlr3misc: https://github.com/mlr-org/mlr3misc/pull/105/files and release
• Merge feat(Learner): allow to strip srcrefs from columns in prototype mlr3#1002 and release
• implement strip_srcref.tfd_reg and strip_srcref.tfd_irreg methods in mlr3fda.