Add internal MLP learner

[ielbadisy]

Summary

• add a torch-backed internal mlp learner for regression, binary classification, and multiclass classification
• expose mlp in the funcml learner registry/catalog and docs
• extend learner contract tests for mlp

Tests

• Rscript -e 'devtools::test(reporter=summary)'
• R CMD build --no-build-vignettes . && R CMD check --no-manual --ignore-vignettes funcml_0.7.2.tar.gz