Fix TensorFlow stdlib module compilation. (#546)

Follow-up to #540 fixing
TensorFlow stdlib module compilation.

- Rename `Sources/Tensor/Utilities.swift -> Sources/Tensor/TensorUtilities.swift`
  to fix stdlib compilation error: `filename "Utilities.swift" used twice`.

- Add `#if !COMPILING_TENSORFLOW_STDLIB_MODULE` guards gating `import Tensor`
declarations so that the TensorFlow module can be built as part of the stdlib.
  - This will no longer be necessary when TensorFlow is no longer built as
    part of the stdlib: #488.

Author: dan-zheng

Sources/Tensor/Utilities.swift renamed to Sources/Tensor/TensorUtilities.swift

@@ -18,7 +18,9 @@ import Darwin
 import Glibc
 #endif
 
+#if !COMPILING_TENSORFLOW_STDLIB_MODULE
 import Tensor
+#endif
 
 /// A value that indicates the phase of using a machine learning model.
 public enum LearningPhase {

Sources/TensorFlow/Context.swift

@@ -12,5 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_STDLIB_MODULE
 // Re-export all public symbols in the Tensor library.
 @_exported import Tensor
+#endif

Sources/TensorFlow/Exports.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_STDLIB_MODULE
 import Tensor
+#endif
 
 public extension Tensor {
     /// Creates a tensor with the specified shape and a single, repeated scalar value.

Sources/TensorFlow/Initializers.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_STDLIB_MODULE
 import Tensor
+#endif
 
 public typealias ParameterInitializer<Scalar: TensorFlowScalar> = (TensorShape) -> Tensor<Scalar>
 

Sources/TensorFlow/Layers/Initialization.swift

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if !COMPILING_TENSORFLOW_STDLIB_MODULE
 import Tensor
+#endif
 
 /// An input to a recurrent neural network.
 public struct RNNCellInput<Input: Differentiable, State: Differentiable>: Differentiable {