Use an arc in the varbuilder rather than rc. (#757)

* Use an arc in the varbuilder rather than rc.

* Require the backends to be send.

* Request send and sync.

candle-nn/src/var_builder.rs

@@ -5,14 +5,14 @@ use crate::VarMap;
 use candle::{safetensors::Load, DType, Device, Error, Result, Shape, Tensor};
 use safetensors::{slice::IndexOp, tensor::SafeTensors};
 use std::collections::HashMap;
-use std::rc::Rc;
+use std::sync::Arc;
 
 /// A structure used to retrieve variables, these variables can either come from storage or be
 /// generated via some form of initialization.
 ///
 /// The way to retrieve variables is defined in the backend embedded in the `VarBuilder`.
 pub struct VarBuilderArgs<'a, B: Backend> {
-    data: Rc<TensorData<B>>,
+    data: Arc<TensorData<B>>,
     path: Vec<String>,
     _phantom: std::marker::PhantomData<&'a B>,
 }
@@ -43,7 +43,7 @@ struct TensorData<B: Backend> {
 /// Note that there is a speciliazed version of this trait (`SimpleBackend`) that can be used most
 /// of the time. The main restriction is that it doesn't allow for specific args (besides
 /// initialization hints).
-pub trait Backend {
+pub trait Backend: Send + Sync {
     type Hints: Default;
 
     /// Retrieve a tensor with some target shape.
@@ -59,7 +59,7 @@ pub trait Backend {
     fn contains_tensor(&self, name: &str) -> bool;
 }
 
-pub trait SimpleBackend {
+pub trait SimpleBackend: Send + Sync {
     /// Retrieve a tensor based on a target name and shape.
     fn get(
         &self,
@@ -99,7 +99,7 @@ impl<'a, B: Backend> VarBuilderArgs<'a, B> {
             device: dev.clone(),
         };
         Self {
-            data: Rc::new(data),
+            data: Arc::new(data),
             path: vec![],
             _phantom: std::marker::PhantomData,
         }
@@ -333,7 +333,7 @@ impl<'a> VarBuilder<'a> {
             device,
         };
         Self {
-            data: Rc::new(data),
+            data: Arc::new(data),
             path: vec![],
             _phantom: std::marker::PhantomData,
         }