feat: add pth varbuilder (#1108)

candle-nn/src/var_builder.rs

@@ -191,6 +191,7 @@ impl<'a, B: Backend> VarBuilderArgs<'a, B> {
 }
 
 struct Zeros;
+
 impl SimpleBackend for Zeros {
     fn get(&self, s: Shape, _: &str, _: crate::Init, dtype: DType, dev: &Device) -> Result<Tensor> {
         Tensor::zeros(s, dtype, dev)
@@ -325,6 +326,39 @@ impl SimpleBackend for candle::npy::NpzTensors {
     }
 }
 
+impl SimpleBackend for candle::pickle::PthTensors {
+    fn get(
+        &self,
+        s: Shape,
+        path: &str,
+        _: crate::Init,
+        dtype: DType,
+        dev: &Device,
+    ) -> Result<Tensor> {
+        let tensor = match self.get(path)? {
+            None => Err(Error::CannotFindTensor {
+                path: path.to_string(),
+            }
+            .bt())?,
+            Some(tensor) => tensor,
+        };
+        let tensor = tensor.to_device(dev)?.to_dtype(dtype)?;
+        if tensor.shape() != &s {
+            Err(candle::Error::UnexpectedShape {
+                msg: format!("shape mismatch for {path}"),
+                expected: s,
+                got: tensor.shape().clone(),
+            }
+            .bt())?
+        }
+        Ok(tensor)
+    }
+
+    fn contains_tensor(&self, name: &str) -> bool {
+        self.get(name).map_or(false, |v| v.is_some())
+    }
+}
+
 impl SimpleBackend for candle::safetensors::MmapedSafetensors {
     fn get(
         &self,
@@ -438,9 +472,16 @@ impl<'a> VarBuilder<'a> {
         let npz = candle::npy::NpzTensors::new(p)?;
         Ok(Self::new(Box::new(npz), dtype, dev.clone()))
     }
+
+    /// Initializes a `VarBuilder` that retrieves tensors stored in a pytorch pth file.
+    pub fn from_pth<P: AsRef<std::path::Path>>(p: P, dtype: DType, dev: &Device) -> Result<Self> {
+        let pth = candle::pickle::PthTensors::new(p)?;
+        Ok(Self::new(Box::new(pth), dtype, dev.clone()))
+    }
 }
 
 pub struct ShardedSafeTensors(candle::safetensors::MmapedSafetensors);
+
 pub type ShardedVarBuilder<'a> = VarBuilderArgs<'a, ShardedSafeTensors>;
 
 impl ShardedSafeTensors {