Fix gradient overflow issue during attention mask

This fix is in reference to issue #382. GPT2 can now be trained in mixed precision, which I've confirmed with testing. I also tested unconditional generation on multiple seeds before and after changing 1e10 to 1e4 and there was no difference. Please let me know if there is anything else I can do to make this pull request better. Thanks for all your work!

pytorch_pretrained_bert/modeling_gpt2.py

@@ -218,7 +218,7 @@ class Attention(nn.Module):
             w = w / math.sqrt(v.size(-1))
         nd, ns = w.size(-2), w.size(-1)
         b = self.bias[:, :, ns-nd:ns, :ns]
-        w = w * b - 1e10 * (1 - b)
+        w = w * b - 1e4 * (1 - b)
 
         w = nn.Softmax(dim=-1)(w)
         return torch.matmul(w, v)