Vision dataset (#179)

* Add some readers for the mnist dataset.

* Import the cifar and mnist dataset.

candle-nn/src/vision/mod.rs

@@ -0,0 +1,12 @@
+use candle::Tensor;
+
+pub struct Dataset {
+    pub train_images: Tensor,
+    pub train_labels: Tensor,
+    pub test_images: Tensor,
+    pub test_labels: Tensor,
+    pub labels: usize,
+}
+
+pub mod cifar;
+pub mod mnist;