Add the candle-datasets crate (#322)

* Move the vision datasets to a separate crate.

* Move the batcher bits.

* Update the readme.

* Move the tiny-stories bits.

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Co-authored-by: Jane Doe <jane.doe@example.org>

candle-datasets/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;