Add precondition message for Tensor initialization (#301)

Author: jon-tow

Sources/TensorFlow/Core/Tensor.swift

@@ -140,6 +140,8 @@ public extension Tensor {
     @inlinable
     @differentiable(wrt: self, vjp: _vjpScalarized where Scalar: TensorFlowFloatingPoint)
     func scalarized() -> Scalar {
+        precondition(shape.contiguousSize == 1,
+           "This tensor must have exactly one scalar but contains \(shape.contiguousSize).")
         return reshaped(to: []).scalar!
     }
 }
@@ -226,9 +228,14 @@ public extension Tensor {
     /// - Parameters:
     ///   - shape: The shape of the tensor.
     ///   - scalars: The scalar contents of the tensor.
-    /// - Precondition: The number of scalars must equal the product of the dimensions of the shape.
+    /// - Precondition: The product of the dimensions of the shape must equal the number of scalars.
     @inlinable
     init(shape: TensorShape, scalars: [Scalar]) {
+        precondition(shape.contiguousSize == scalars.count,
+            """
+            The shape requires \(shape.contiguousSize) scalars but \(scalars.count) were \
+            provided.
+            """)
         self = scalars.withUnsafeBufferPointer { bufferPointer in
 	        Tensor(shape: shape, scalars: bufferPointer)
 	    }
@@ -239,11 +246,14 @@ public extension Tensor {
     /// - Parameters:
     ///   - shape: The shape of the tensor.
     ///   - scalars: The scalar contents of the tensor.
-    /// - Precondition: The number of scalars must equal the product of the
-    ///   dimensions of the shape.
+    /// - Precondition: The product of the dimensions of the shape must equal the number of scalars.
     @inlinable
     init(shape: TensorShape, scalars: UnsafeBufferPointer<Scalar>) {
-        precondition(scalars.count == shape.contiguousSize)
+        precondition(shape.contiguousSize == scalars.count,
+            """
+            The shape requires \(shape.contiguousSize) scalars but \(scalars.count) were \
+            provided.
+            """)
         let handle = TensorHandle<Scalar>(
             shape: shape.dimensions,
             scalarsInitializer: { address in
@@ -257,11 +267,14 @@ public extension Tensor {
     /// - Parameters:
     ///   - shape: The shape of the tensor.
     ///   - scalars: The scalar contents of the tensor.
-    /// - Precondition: The number of scalars must equal the product of the
-    ///   dimensions of the shape.
+    /// - Precondition: The product of the dimensions of the shape must equal the number of scalars.
     @inlinable
     init<C: RandomAccessCollection>(shape: TensorShape, scalars: C) where C.Element == Scalar {
-        precondition(scalars.count == shape.contiguousSize)
+        precondition(shape.contiguousSize == scalars.count,
+            """
+            The shape requires \(shape.contiguousSize) scalars but \(scalars.count) were \
+            provided.
+            """)
         let handle = TensorHandle<Scalar>(
             shape: shape.dimensions,
             scalarsInitializer: { addr in