Blocks deletion from already deleted heads. Necessary integration test.

Now raises a warning when a head to be deleted already has been deleted. An integration test verifying the total pipeline (-> from config -> save model -> load model -> additional head pruning) has been added.

pytorch_transformers/modeling_xlm.py

@@ -561,6 +561,7 @@ class XLMModel(XLMPreTrainedModel):
 
         if hasattr(config, "pruned_heads"):
             pruned_heads = config.pruned_heads.copy().items()
+            config.pruned_heads = {}
             for layer, heads in pruned_heads:
                 if self.attentions[int(layer)].n_heads == config.n_heads:
                     self.prune_heads({int(layer): list(map(int, heads))})