Allow to skip stage 2 (i.e. max_iter_2=0)

Running the imperceptible attack without stage 2 equals the Carlini and
Wagner audio attack (see paper link: https://arxiv.org/abs/1801.01944).

Signed-off-by: Hessel Tuinhof <[email protected]>

Author: hesseltuinhof

art/attacks/evasion/imperceptible_asr/imperceptible_asr.py

@@ -494,8 +494,8 @@ def _check_params(self) -> None:
 
         if not isinstance(self.max_iter_2, int):
             raise ValueError("The maximum number of iterations for stage 2 must be of type int.")
-        if self.max_iter_2 <= 0:
-            raise ValueError("The maximum number of iterations for stage 2 must be greater than 0.")
+        if self.max_iter_2 < 0:
+            raise ValueError("The maximum number of iterations for stage 2 must be non-negative.")
 
         if not isinstance(self.learning_rate_1, float):
             raise ValueError("The learning rate for stage 1 must be of type float.")