Fix crash in einsum

• Fix crash in einsum where a lack of expression for inner reduce causes computing outer expression to crash.

hercules_ir/src/einsum.rs

@@ -235,12 +235,11 @@ impl<'a> EinsumContext<'a> {
                 control: _,
                 init: _,
                 reduct: _,
-            } => {
-                let reduce = self.so_far[&id];
+            } if let Some(reduce) = self.so_far.get(&id) => {
                 // Substitute opaque uses of thread ID nodes in inner expression
                 // with thread ID math expression, and increment inner-fork
                 // dimensions (alpha renaming).
-                return self.substitute_new_dims(reduce);
+                return self.substitute_new_dims(*reduce);
             }
             _ => MathExpr::OpaqueNode(id),
         };