Fix cyclic import

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

art/attacks/poisoning/sleeper_agent_attack.py

@@ -29,7 +29,7 @@
 import numpy as np
 from tqdm.auto import trange
 
-from art.attacks.poisoning import GradientMatchingAttack
+from art.attacks.poisoning.gradient_matching_attack import GradientMatchingAttack
 from art.estimators.classification.pytorch import PyTorchClassifier
 from art.estimators.classification import TensorFlowV2Classifier
 from art.preprocessing.standardisation_mean_std.pytorch import StandardisationMeanStdPyTorch