fix batch_shape for point approximator

Author: arrjon

bayesflow/approximators/continuous_approximator.py

@@ -536,8 +536,12 @@ def _sample(
                 inference_conditions, (batch_size, num_samples, *keras.ops.shape(inference_conditions)[2:])
             )
 
-            target_dim = self.inference_network.base_distribution.dims
-            batch_shape = keras.ops.shape(inference_conditions)[: -len(target_dim)]
+            if hasattr(self.inference_network, "base_distribution"):
+                target_shape_len = len(self.inference_network.base_distribution.dims)
+            else:
+                # point approximator has no base_distribution
+                target_shape_len = 1
+            batch_shape = keras.ops.shape(inference_conditions)[:-target_shape_len]
         else:
             batch_shape = (num_samples,)