[WordSeg] Clarify autodiff workarounds. (#521)
[WordSeg] Clarify autodiff workarounds. (#521)

Clarify autodiff workarounds related to TF-433: autodiff does not yet support
non-active throwing/rethrowing functions like `Array.map` and `Bool.&&`.
tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models