Make derivatives of public functions be @usableFromInline.

Author: dan-zheng

Sources/TensorFlow/Core/MixedPrecision.swift

@@ -206,11 +206,13 @@ extension Tensor {
 #endif
 
 extension Tensor where Scalar: TensorFlowFloatingPoint {
+  @usableFromInline
   @derivative(of: toReducedPrecision)
   func _vjpToReducedPrecision() -> (value: Tensor, pullback: (Tensor) -> Tensor) {
     (toReducedPrecision, { $0.toFullPrecision })
   }
 
+  @usableFromInline
   @derivative(of: toFullPrecision)
   func _vjpToFullPrecision() -> (value: Tensor, pullback: (Tensor) -> Tensor) {
     (toFullPrecision, { $0.toReducedPrecision })

Sources/TensorFlow/Layer.swift

@@ -94,8 +94,8 @@ extension Layer {
 
   // TODO(TF-433, SR-11882): Remove this custom derivative when
   // differentiation supports `rethrows` functions and currying.
-  @derivative(of: inferring(from:))
   @usableFromInline
+  @derivative(of: inferring(from:))
   internal func _vjpInferring(from input: Input)
     -> (
       value: Output,