Bug fix for standalone mod switch down for CKKS

* Bug fix for standalone mod switch for CKKS. Previous version of the code rescales after reducing the RNS, which is not required. Requires refactoring in the future.

---------

Co-authored-by: Jose Rojas Chaves <jose.rojas.chaves@intel.com>

Author: christopherngutierrez

p-isa_tools/kerngen/pisa_generators/mod.py

@@ -9,6 +9,7 @@
 from high_parser import KernelContext, Immediate, HighOp, Polys
 
 from .basic import (
+    Copy,
     Add,
     Sub,
     Muli,
@@ -53,6 +54,12 @@ def to_pisa(self) -> list[PIsaOp]:
             self.input0, self.context.current_rns
         )
 
+        # For CKKS and standalone mod switch down, we can reduce the RNS by one and copy to output, no need to rescale
+        if self.context.scheme == "CKKS" and self.var_suffix == self.MOD_QLAST:
+            return mixed_to_pisa_ops(
+                [Copy(self.context, self.output, input_remaining_rns)]
+            )
+
         # Temp.
         temp_input_last_rns = duplicate_polys(input_last_rns, "y")
         temp_input_remaining_rns = duplicate_polys(input_remaining_rns, "x")