Detach the tensors on batch-norm eval. (#1702)

* Detach the tensors on batch-norm eval.

* Fix pyo3 bindings.

* Black tweak.

* Formatting.

* Also update the pyo3-onnx formatting.

* Apply black.

candle-core/src/backprop.rs

@@ -175,7 +175,7 @@ impl Tensor {
             // the backprop graph of the backprop itself. This would be an issue for second order
             // derivatives but these are out of scope at the moment.
             let do_not_detach = CANDLE_GRAD_DO_NOT_DETACH.with(|b| *b);
-            let grad = if do_not_detach { grad } else { grad.detach()? };
+            let grad = if do_not_detach { grad } else { grad.detach() };
             if let Some(op) = node.op() {
                 match op {
                     Op::Binary(lhs, rhs, BinaryOp::Add) => {

candle-core/src/tensor.rs

@@ -1882,9 +1882,9 @@ impl Tensor {
     /// 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> {
+    pub fn detach(&self) -> Tensor {
         if self.op.is_none() && !self.is_variable {
-            Ok(self.clone())
+            self.clone()
         } else {
             let tensor_ = Tensor_ {
                 id: TensorId::new(),
@@ -1895,7 +1895,7 @@ impl Tensor {
                 dtype: self.dtype,
                 device: self.device.clone(),
             };
-            Ok(Tensor(Arc::new(tensor_)))
+            Tensor(Arc::new(tensor_))
         }
     }
 

candle-core/src/variable.rs

@@ -107,6 +107,10 @@ impl Var {
         Ok(Self(inner))
     }
 
+    pub fn as_detached_tensor(&self) -> Tensor {
+        self.0.detach()
+    }
+
     pub fn as_tensor(&self) -> &Tensor {
         &self.0
     }