Add the permute op (similar to pytorch). (#504)

* Add the permute op (similar to pytorch).

* Add the backprop for dimension permutation.

candle-core/src/op.rs

@@ -117,6 +117,7 @@ pub enum Op {
     Reshape(Tensor),
     ToDevice(Tensor),
     Transpose(Tensor, usize, usize),
+    Permute(Tensor, Vec<usize>),
     Elu(Tensor, f64),
     CustomOp1(Tensor, std::sync::Arc<Box<dyn CustomOp1 + Send + Sync>>),
     CustomOp2(