update usage of deprecated AD APIs

[marcrasi]

This corresponds to https://github.com/fastai/fastai_dev/pull/311.

• @nondiff -> @noDerivative
• @differentiable(vjp:) -> @derivative(of:)
• model.valueWithGradient -> valueWithGradient(at: model)
• Update links that referred to no-longer-existing Ops.swift file.
• Delete custom derivative of FALayer.callAsFunction because control flow AD can understand the callback loop now.