correct _vjpRsqrt to match Raw.rsqrtGrad (#24888)

mikowals · rxwei · commit 1261704a899b · 2019-05-18T04:05:14.000-07:00
correct _vjpRsqrt to match Raw.rsqrtGrad I got the updated derivative from here - https://www.derivative-calculator.net - and this matches the result of Raw.rsqrtGrad. I have just created the pull request by clicking the edit button in GitHub so I can not confirm that it builds or that tests pass.
diff --git a/stdlib/public/TensorFlow/Gradients.swift b/stdlib/public/TensorFlow/Gradients.swift
@@ -455,7 +455,7 @@ func _vjpRsqrt<T : TensorFlowFloatingPoint>(
   _ x: Tensor<T>
 ) -> (Tensor<T>, (Tensor<T>) -> Tensor<T>) {
   let value = rsqrt(x)
-  return (value, { v in -v / 2 * value })
+  return (value, { v in -v / (2 * pow(x, 3 / 2))})
 }
 
 @inlinable

Original file line number	Diff line number	Diff line change
`@@ -455,7 +455,7 @@ func _vjpRsqrt<T : TensorFlowFloatingPoint>(`
`455`	`455`	`_ x: Tensor<T>`
`456`	`456`	`) -> (Tensor<T>, (Tensor<T>) -> Tensor<T>) {`
`457`	`457`	`let value = rsqrt(x)`
`458`		`- return (value, { v in -v / 2 * value })`
	`458`	`+ return (value, { v in -v / (2 * pow(x, 3 / 2))})`
`459`	`459`	`}`
`460`	`460`
`461`	`461`	`@inlinable`