Make library resilient.

Mark layer structs as `@_fixed_layout`.
Gate TensorFlow module import on `COMPILING_TENSORFLOW_MODULE` flag.
This will be used during stdlib compilation to prevent dubious import.

Author: dan-zheng

Sources/DeepLearning/Helpers.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_MODULE
 import TensorFlow
+#endif
 
 // `pow` is defined in Darwin/Glibc on `Float` and `Double`, but there doesn't exist a generic
 // version for `FloatingPoint`.

Sources/DeepLearning/Layer.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_MODULE
 @_exported import TensorFlow
+#endif
 
 /// A neural network layer.
 ///
@@ -44,6 +46,7 @@ public extension Layer {
     }
 }
 
+@_fixed_layout
 public struct Dense<Scalar>: Layer
     where Scalar : FloatingPoint & Differentiable & TensorFlowScalar {
     public var weight: Tensor<Scalar>
@@ -63,6 +66,7 @@ public extension Dense where Scalar : BinaryFloatingPoint,
     }
 }
 
+@_fixed_layout
 public struct Conv2D<Scalar>: Layer
     where Scalar : FloatingPoint & Differentiable & TensorFlowScalar {
     public var filter: Tensor<Scalar>

Sources/DeepLearning/Loss.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_MODULE
 import TensorFlow
+#endif
 
 @differentiable(vjp: _vjpMSE)
 public func meanSquaredError<Scalar: FloatingPoint>(predicted: Tensor<Scalar>,

Sources/DeepLearning/Optimizer.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_MODULE
 import TensorFlow
+#endif
 
 public protocol Optimizer {
     associatedtype Model: Layer