[WordSeg] Fix pullback for `Array.update(at:to:)`. (#653)
[WordSeg] Fix pullback for `Array.update(at:to:)`. (#653)

The pullback of `Array.update(at:to:)` resets `dSelf.base[index]` to a zero value.

For correctness, it should use the value of `self[index].zeroTangentVector` before
the update occurs, not `dElement.zeroTangentVector`.
tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models