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_gpt2.py

@@ -455,6 +455,7 @@ class GPT2Model(GPT2PreTrainedModel):
 
         if hasattr(config, "pruned_heads"):
             pruned_heads = config.pruned_heads.copy().items()
+            config.pruned_heads = {}
             for layer, heads in pruned_heads:
                 if self.h[int(layer)].attn.n_head == config.n_head:
                     self.prune_heads({int(layer): list(map(int, heads))})