Enhance `Layer` methods and make backward-compatible.
Enhance `Layer` methods and make backward-compatible.

* Add doc comments to everything.

* Introduce `inferring(from:)`, which is equivalent to `applied(to:in:)` with a default inference context.

* Add back `applied(to:)` that simply calls `inferring(from:)`. Add a deprecation warning that tells people to switch to either `applied(to:in:)` or `inferring(from:)`.

* Rename `Layer.valueWithPullback(at:in:)` to `appliedForBackpropagation(to:in:)` to ease the learning curve.

tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis