[feat] Trait for Circuit with Default Inputs (#223)

* Add a trait for Circuit with default inputs

* Fix lint

* Remove witness_gen_only

halo2-base/src/utils/halo2.rs

@@ -3,7 +3,7 @@ use std::collections::hash_map::Entry;
 use crate::ff::Field;
 use crate::halo2_proofs::{
     circuit::{AssignedCell, Cell, Region, Value},
-    plonk::{Advice, Assigned, Column, Fixed},
+    plonk::{Advice, Assigned, Circuit, Column, Fixed},
 };
 use crate::virtual_region::copy_constraints::{CopyConstraintManager, EXTERNAL_CELL_TYPE_ID};
 use crate::AssignedValue;
@@ -106,3 +106,9 @@ pub fn constrain_virtual_equals_external<F: Field + Ord>(
         }
     }
 }
+
+/// Circuit with default values.
+pub trait CircuitWithDefault<F: Field>: Circuit<F> {
+    /// Return Circuit with default inputs for the given params. Return struct could be used for pkey generation.
+    fn default(params: Self::Params) -> Self;
+}