Lazy detach. (#1242)

candle-core/src/op.rs

@@ -920,6 +920,10 @@ impl BackpropOp {
         };
         Self(op)
     }
+
+    pub(crate) fn is_none(&self) -> bool {
+        self.0.is_none()
+    }
 }
 
 impl std::ops::Deref for BackpropOp {

candle-core/src/tensor.rs

@@ -1807,7 +1807,12 @@ impl Tensor {
 
     /// Returns a new tensor detached from the current graph, gradient are not propagated through
     /// this new node. The storage of this tensor is shared with the initial tensor.
+    ///
+    /// If the tensor is already detached from the computation graph, the same tensor is returned.
     pub fn detach(&self) -> Result<Tensor> {
+        if self.op.is_none() && !self.is_variable {
+            Ok(self.clone())
+        } else {
             let tensor_ = Tensor_ {
                 id: TensorId::new(),
                 storage: self.storage.clone(),
@@ -1819,6 +1824,7 @@ impl Tensor {
             };
             Ok(Tensor(Arc::new(tensor_)))
         }
+    }
 
     /// If the target device is the same as the tensor device, only a shallow copy is performed.
     pub fn to_device(&self, device: &Device) -> Result<Tensor> {