public Step — parametric type
Step{T, S<:Integer}(start, decay, step_sizes)
Step(;λ, γ, step_sizes)
A step schedule decays exponentially by γ every step
in step_sizes.
The output conforms to
λ * γ^{i - 1}
where sum(step_sizes[1:(i - 1)]) < t <= sum(step_sizes[1:i])
Arguments:
start/λ: the starting valuedecay/γ: the decay ratestep_sizes::Union{<:Integer, <:Vector}: the step sizes