change masked_bias to -inf
# What does this PR do?

change masked_bias to -inf


## Before submitting
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- [x] Did you write any new necessary tests?


Hi, thank you for opening a PR!

Our goal is to stay as close to the initial implementation as possible. The original implementation by OpenAI uses -1e4, so we will keep it this way.<|||||>I find the initial implementation is `-1e10` in https://github.com/openai/gpt-2/blob/master/src/model.py#L88

```py
w = w*b - tf.cast(1e10, w.dtype)*(1-b)
```
related issue #9594

I am not quite sure, I guess `1e-10` is not compatible with `fp16`, that may be the reason behind huggingface implementation.
