PegasusTokenizer: Newline symbol
Ported models generate the `<n>` token at the beginning of sentences, whereas ours do not. The pegasus [original code](https://github.com/google-research/pegasus/blob/master/pegasus/ops/public_parsing_ops.py#L40) replaces `\n` newline symbol with `<n>`. `PegasusTokenizer` should probably do this. 

```python
_NEWLINE_SYMBOL = "<n>"
text = tf.strings.regex_replace(text, "\n", _NEWLINE_SYMBOL)
```


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