Fix gradient overflow issue during attention mask
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!
huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers