Issue Description:

The following code can run with swiftc -ltensorflow, but not swift only.

import TensorFlow
import Python

let np = Python.import("numpy")

func readFile(_ filename: String) -> [UInt8] {
let d = Python.open(filename, "rb").read()
return Array(numpyArray: np.frombuffer(d, dtype: np.uint8))!
}

func readMNIST(imagesFile: String, labelsFile: String) ->
(images: Tensor<Float>, labels: Tensor<Int32>) {
// print("Reading data.")

let images = readFile(imagesFile).dropFirst(16).map { Float($0) }
let labels = readFile(labelsFile).dropFirst(8).map { Int32($0) }
let rowCount = Int32(labels.count)
let columnCount = Int32(images.count) / rowCount

// // print("Constructing data tensors.")
let imagesTensor = Tensor(shape: [rowCount, columnCount], scalars: images) / 255
let labelsTensor = Tensor(labels)
return (imagesTensor, labelsTensor)
}

func train() {
// Get training data.
let (images, numericLabels) = readMNIST(imagesFile: "train-images-idx3-ubyte",
labelsFile: "train-labels-idx1-ubyte")
print(images.shape)
print(numericLabels.shape)
}

train()