Provide a method to allow PTH files with state maps to be loaded. (#2639)

* Provide a method to allow PTH files iwth state maps to be loaded.

* add a line to the doc

* String-. &str

candle-nn/src/var_builder.rs

@@ -544,7 +544,17 @@ impl<'a> VarBuilder<'a> {
         let pth = candle::pickle::PthTensors::new(p, None)?;
         Ok(Self::from_backend(Box::new(pth), dtype, dev.clone()))
     }
-
+    /// Initializes a `VarBuilder` that retrieves tensors stored in a pytorch pth file.
+    /// similar to [`from_pth`] but requires a `state_key`.
+    pub fn from_pth_with_state<P: AsRef<std::path::Path>>(
+        p: P,
+        dtype: DType,
+        state_key: &str,
+        dev: &Device,
+    ) -> Result<Self> {
+        let pth = candle::pickle::PthTensors::new(p, Some(state_key))?;
+        Ok(Self::from_backend(Box::new(pth), dtype, dev.clone()))
+    }
     /// Gets a VarBuilder that applies some renaming function on tensor it gets queried for before
     /// passing the new names to the inner VarBuilder.
     ///