VarMap setter functions (#938)

* Add some setter helper functions for varmap.

* Add more comments.

candle-nn/src/var_map.rs

@@ -57,6 +57,44 @@ impl VarMap {
         Ok(())
     }
 
+    /// Set a named variable to some value.
+    pub fn set_one<K: AsRef<String>, V: AsRef<Tensor>>(&mut self, name: K, value: V) -> Result<()> {
+        let tensor_data = self.data.lock().unwrap();
+        let name = name.as_ref();
+        match tensor_data.get(name) {
+            None => candle::bail!("cannot find {name} in VarMap"),
+            Some(var) => {
+                if let Err(err) = var.set(value.as_ref()) {
+                    candle::bail!("error setting {name}: {err}",)
+                }
+            }
+        }
+        Ok(())
+    }
+
+    /// Set some named variables to some values.
+    ///
+    /// If an error is returned, some of the variables might have already been set to their new
+    /// values.
+    pub fn set<I: Iterator<Item = (K, V)>, K: AsRef<String>, V: AsRef<Tensor>>(
+        &mut self,
+        iter: I,
+    ) -> Result<()> {
+        let tensor_data = self.data.lock().unwrap();
+        for (name, value) in iter {
+            let name = name.as_ref();
+            match tensor_data.get(name) {
+                None => candle::bail!("cannot find {name} in VarMap"),
+                Some(var) => {
+                    if let Err(err) = var.set(value.as_ref()) {
+                        candle::bail!("error setting {name}: {err}",)
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
     /// Retrieve or add a new variable.
     pub fn get<S: Into<Shape>>(
         &self,