Add AudioLDM2 TTS (#5381)
* add audioldm2 tts
* change gpt2 max new tokens
* remove unnecessary pipeline and class
* add TTS to AudioLDM2Pipeline
* add TTS docs
* delete unnecessary file
* remove unnecessary import
* add audioldm2 slow testcase
* fix code quality
* remove AudioLDMLearnablePositionalEmbedding
* add variable check vits encoder
* add use_learned_position_embedding
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Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>
