Internalize signature delegates in Core and Data assemblies

[TomFinley]

We have these "signature" delegates throughout the codebase, used for purposes of entry-points and other dependency injection style things.

Many are public, as we see here.

machinelearning/src/Microsoft.ML.Data/Dirty/ILoss.cs

Line 49 in 7e21afa

public delegate void SignatureClassificationLoss();

Yet there is no need for them to be public, and many of these have already been "internalized," e.g.:

machinelearning/src/Microsoft.ML.Core/Prediction/ITrainer.cs

Line 20 in 7e21afa

internal delegate void SignatureBinaryClassifierTrainer();

We should complete this work, since we do not want these as part of the public surface of the API.

[abgoswam]

@TomFinley . Can I assign this to myself and start working on this ? (I don't see the up-for-grabs label, hence asking)

[Ivanidzo4ka]

We have plenty of Lockdown assembly issues, and I assume internalization of delegates in that assemblies would happen automatically. But we don't have "lockdown" issues for Core and Data assembly, so I'm renaming this issue to reflect scope of work.