Get the comparison operation to work on scalar values. (#780)

* Get the comparison operation to work on scalar values.

* Add some time measurement.

candle-core/src/scalar.rs

@@ -0,0 +1,23 @@
+use crate::{Result, Tensor, WithDType};
+
+pub enum TensorScalar {
+    Tensor(Tensor),
+    Scalar(Tensor),
+}
+
+pub trait TensorOrScalar {
+    fn to_tensor_scalar(self) -> Result<TensorScalar>;
+}
+
+impl TensorOrScalar for &Tensor {
+    fn to_tensor_scalar(self) -> Result<TensorScalar> {
+        Ok(TensorScalar::Tensor(self.clone()))
+    }
+}
+
+impl<T: WithDType> TensorOrScalar for T {
+    fn to_tensor_scalar(self) -> Result<TensorScalar> {
+        let scalar = Tensor::new(self, &crate::Device::Cpu)?;
+        Ok(TensorScalar::Scalar(scalar))
+    }
+}