Add the slice indexing.

tests/tensor_tests.rs

@@ -9,5 +9,7 @@ fn add() -> Result<()> {
     let tensor = Tensor::new([3f32, 1., 4.].as_slice(), Device::Cpu)?;
     let dim1 = tensor.shape().r1()?;
     assert_eq!(dim1, 3);
+    let content: Vec<f32> = tensor.to_vec1()?;
+    assert_eq!(content, [3., 1., 4.]);
     Ok(())
 }