Flashlight improvements and fixes, also fixed potential VRAM leak in ctc_models.transcribe
# What does this PR do ?

Flashlight improvements and fixes, also fixed potential VRAM leak in ctc_models.transcribe

**Collection**: ASR

# Changelog 
- Set logits to CPU in ctc_models.transcribe
- Changed all `self.decoder.vocabulary` calls in ctc_models to be `OmegaConf.to_container(self.decoder.vocabulary)`, this is necessary for performance reasons, because downstream decoders like Flashlight access the vocabulary a lot, and if it's passed as a ListConfig, it's incredibly expensive to access repeatedly, hence pass as Python list instead
- Made changes to silence/unknown token handling in flashlight decoder
- Removed the unit_lm boolean requirement in flashlight_cfg. It's not necessary, the decoder knows that if you pass a lexicon it should use it, otherwise do lexicon-free mode. There's no need for a boolean too, it's superfluous, the presence of the lexicon file is the only indicator the class needs

# Usage
No changes to how anything is used, except don't pass `unit_lm` anymore to `flashlight_cfg`

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- [X] Bugfix
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# Additional Information
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