Add optional for exceptions to satisfy pytype (#6495)

bindings/python/iree/runtime/function.py

diff --git a/bindings/python/iree/runtime/function.py b/bindings/python/iree/runtime/function.py
index 36a5280..e46b29f 100644
--- a/bindings/python/iree/runtime/function.py
+++ b/bindings/python/iree/runtime/function.py
@@ -381,7 +381,9 @@
   return dtype(x)
 
 
-def _raise_argument_error(inv: Invocation, summary: str, e: Exception = None):
+def _raise_argument_error(inv: Invocation,
+                          summary: str,
+                          e: Optional[Exception] = None):
   new_e = ValueError(f"Error passing argument: {summary} "
                      f"(while encoding argument {inv.summarize_arg_error()})")
   if e:
@@ -390,7 +392,9 @@
     raise new_e
 
 
-def _raise_return_error(inv: Invocation, summary: str, e: Exception = None):
+def _raise_return_error(inv: Invocation,
+                        summary: str,
+                        e: Optional[Exception] = None):
   new_e = ValueError(f"Error processing function return: {summary} "
                      f"(while decoding return {inv.summarize_return_error()})")
   if e: