Bug fix in MP3 and FLAC compute length on TTSDataset (#3092)

* Bug Fix on XTTS load

* Bug fix in MP3 length on TTSDataset

* Update TTS/tts/datasets/dataset.py

Co-authored-by: Aarni Koskela <akx@iki.fi>

* Uses mutagen for all audio formats

* Add dataloader test wit hall supported audio formats

* Use mutagen.File

* Update

* Fix aux unit tests

* Bug fixe on unit tests

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Co-authored-by: Aarni Koskela <akx@iki.fi>