MNIST images should have an extra dim (pytorch#13060)

Summary:
Our convolution ops and such expect three dimensional images, but the images in the MNIST dataset of the C++ frontend currently only have two.

apaszke ebetica soumith
Pull Request resolved: pytorch#13060

Differential Revision: D10560754

Pulled By: goldsborough

fbshipit-source-id: a2cc877b4f43434482bec902c941fafb7a157d5d

torch/csrc/api/src/data/datasets/mnist.cpp

@@ -73,7 +73,8 @@ Tensor read_images(const std::string& root, bool train) {
   expect_int32(images, kImageRows);
   expect_int32(images, kImageColumns);
 
-  auto tensor = torch::empty({count, kImageRows, kImageColumns}, torch::kByte);
+  auto tensor =
+      torch::empty({count, 1, kImageRows, kImageColumns}, torch::kByte);
   images.read(reinterpret_cast<char*>(tensor.data_ptr()), tensor.numel());
   return tensor.to(torch::kFloat32).div_(255);
 }