Add L2/L1 regularization

[ablaom]

This PR:

• resolves #169
• moves responsibility for CPU <--> GPU data movement to the fit/update methods. Previously, this was done in the fit! method. This change was natural in addressing above and anticipates further transfer of responsibility to planned data front-end (see https://alan-turing-institute.github.io/MLJ.jl/dev/adding_models_for_general_use/#Implementing-a-data-front-end and #97).
• adds missing doc-strings to all the models

@ToucheSir Further to Julia Discourse discussion, no change was actually necessary to the coretrain! loop to add regularization. It's just that the loss function passed to this loop now depends on the chain (Flux model), when L2/L1 regularisation parameters are non-trivial. To avoid the array mutation error I needed to avoid broadcasting in the computation of the penalty here. Any performance suggestions re these two bits of code appreciated.

[codecov-commenter]

Codecov Report

Merging #179 (67409a5) into dev (a40bdfd) will decrease coverage by 0.14%. The diff coverage is 95.12%.

@@            Coverage Diff             @@
##              dev     #179      +/-   ##
==========================================
- Coverage   90.74%   90.59%   -0.15%     
==========================================
  Files           8        9       +1     
  Lines         216      234      +18     
==========================================
+ Hits          196      212      +16     
- Misses         20       22       +2     

Impacted Files	Coverage Δ
src/types.jl	85.71% <0.00%> (-14.29%)	:arrow_down:
src/common.jl	96.77% <100.00%> (+0.22%)	:arrow_up:
src/core.jl	90.00% <100.00%> (-0.79%)	:arrow_down:
src/penalized_losses.jl	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update a40bdfd...67409a5. Read the comment docs.

[ablaom]

@ayush-1506 Would you like to review this PR? I've tested it locally on a GPU.

@DilumAluthge How do I put the GPU tests back for PR's onto dev?

[ayush-1506]

@ablaom Sure, please give me a day.