Add bos and eos to gpt2 tokenizer
Fix https://github.com/huggingface/transformers/issues/3311

Test:
```
tokenizer = GPT2Tokenizer.from_pretrained('gpt2-medium')
tokenizer.add_special_tokens({'pad_token': '<pad>'})
inputs = tokenizer.batch_encode_plus(["My cute dog", "My cute dog is is"], add_special_tokens=True, padding=True, return_tensors="pt")
```

output:
before:
```
{'input_ids': tensor([[ 3666, 13779,  3290, 50257, 50257],
        [ 3666, 13779,  3290,   318,   318]]), 'attention_mask': tensor([[1, 1, 1, 0, 0],
        [1, 1, 1, 1, 1]])}
```

after:
```
{'input_ids': tensor([[50256,  3666, 13779,  3290, 50256, 50257, 50257],
        [50256,  3666, 13779,  3290,   318,   318, 50256]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 0, 0],
        [1, 1, 1, 1, 1, 1, 1]])}
```
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
