EncDecTransfModelBPE model has bugs in transcribe() method 
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**Describe the bug**

1. `setting return_hypotheses=True` in EncDecTransfModelBPE.transcribe() will crash because of undefined local variables ([here](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/asr/models/transformer_bpe_models.py#L261))
2.  EncDecTransfModelBPE.transcribe() has different APIs from other ASR models, causing `transcripbe_speech.py` to crash. (to be fixed in #8087 )

