feat: add learners conversion for pipeop and graph

[codecov-commenter]

Codecov Report

Merging #704 (439d80d) into master (d41fcc9) will decrease coverage by 0.02%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #704      +/-   ##
==========================================
- Coverage   95.21%   95.18%   -0.03%     
==========================================
  Files          87       87              
  Lines        3363     3365       +2     
==========================================
+ Hits         3202     3203       +1     
- Misses        161      162       +1     

Impacted Files	Coverage Δ
R/GraphLearner.R	94.69% <50.00%> (-0.81%)	:arrow_down:

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[mb706]

I wasn't aware of as_learners, but if this is the intended behaviour, wouldn't it make more sense to have as_learners.default just call as_learner in every case? (see https://github.com/mlr-org/mlr3/issues/865)

[mb706]

(nice work otherwise, though, if @mllg says this shouldn't be in mlr3 we can merge)