Merge pull request #1054 from Trusted-AI/development_issue_1053

Update get_activations of TensorFlowV2Classifier

Author: beat-buesser

art/estimators/classification/tensorflow.py

@@ -1301,9 +1301,10 @@ def get_activations(
                         i_layer = i_name
                         break
             elif isinstance(layer, int):
-                if layer < 0 or layer >= len(self.layer_names):
+                if layer < -len(self.layer_names) or layer >= len(self.layer_names):
                     raise ValueError(
-                        "Layer index %d is outside of range (0 to %d included)." % (layer, len(self.layer_names) - 1)
+                        "Layer index %d is outside of range (-%d to %d)."
+                        % (layer, len(self.layer_names), len(self.layer_names) - 1)
                     )
                 i_layer = layer
             else: