Add fix for the case that prune wrapper is added but no layer is prunable.

PiperOrigin-RevId: 358303341

Author: tensorflower-gardener

tensorflow_model_optimization/python/core/sparsity/keras/pruning_callbacks.py

@@ -64,6 +64,8 @@ def __init__(self):
   def on_train_begin(self, logs=None):
     # Collect all the prunable layers in the model.
     self.prunable_layers = _collect_prunable_layers(self.model)
+    if not self.prunable_layers:
+      return
     # If the model is newly created/initialized, set the 'pruning_step' to 0.
     # If the model is saved and then restored, do nothing.
     if self.prunable_layers[0].pruning_step == -1: