Implement index macro

Signed-off-by: Julius Koskela <julius.koskela@unikie.com>

src/lib.rs

@@ -27,7 +27,7 @@ macro_rules! shape {
 
 #[macro_export]
 macro_rules! index {
-    ($array:expr) => {
-        TensorIndex::from($array)
+    ($tensor:expr, $indices:expr) => {
+        TensorIndex::from(($tensor.shape().clone(), $indices))
     };
 }