    def _clean_text(self, text):
        """
        Performs invalid character removal and whitespace cleanup on text.
        """
        output = []
        for char in text:
            cp = ord(char)
            if cp == 0 or cp == 0xfffd or _is_control(char):
                continue
            if _is_whitespace(char):
                output.append(" ")
            else:
                output.append(char)
        return "".join(output)
Performs invalid character removal and whitespace cleanup on text.
ptstructure/vqa/pytorchnlp/transformers/bert/tokenizer.py