Validation loss logger

[axsk]

This is long overdue.

We have the iso.loggers being called in the iso run! functions.
We should have a

struct ValidationLogger{T}
  logevery::Int
  loss::Vector{Float64}
  iter::Vector{Int}
  validationdata::T
end

which computes the loss on validationdata every logevery calls and stores it, together with the length of the current iso.losses (to align them in plots).

[axsk]

implemented in 7ece9d1