Register DynamicBackdoorGAN in poisoning __init__.py

Added import for DynamicBackdoorGAN in `art.attacks.poisoning.__init__.py` to expose the attack via:

    from art.attacks.poisoning import DynamicBackdoorGAN

This ensures that the new dynamic backdoor poisoning attack is accessible at the package level and consistent with other ART poisoning attacks.

Signed-off-by: Prachi Panwar <[email protected]>

Author: prachi0606

art/attacks/poisoning/__init__.py

@@ -19,3 +19,5 @@
 from art.attacks.poisoning.hidden_trigger_backdoor.hidden_trigger_backdoor_pytorch import HiddenTriggerBackdoorPyTorch
 from art.attacks.poisoning.hidden_trigger_backdoor.hidden_trigger_backdoor_keras import HiddenTriggerBackdoorKeras
 from art.attacks.poisoning.sleeper_agent_attack import SleeperAgentAttack
+from art.attacks.poisoning.dynamic_backdoor_gan import DynamicBackdoorGAN
+