Bug: intersperse_blank_char method uses pad character instead of pad_id

[coderabbitai]

In the intersperse_blank_char method of TTSTokenizer class, char_to_use incorrectly uses self.characters.pad instead of self.characters.pad_id when use_blank_char is False.

This issue is being tracked but not fixed immediately to maintain alignment with coqui-TTS upstream code.

Current code:

char_to_use = self.characters.blank_id if use_blank_char else self.characters.pad

Expected code:

char_to_use = self.characters.blank_id if use_blank_char else self.characters.pad_id

Originally discovered in: //pull/10#discussion_r1921417115