Key Error when using conformer-transducer-bpe
Hi,
I am trying to train a conformer-transducer ASR model with BERT tokenizer. I am using  `conformer_transducer_bpe.yaml` as the base of my config with few changes to set the tokenizer. When running it, I get the following error:
```
    raise ex.with_traceback(sys.exc_info()[2])  # set end OC_CAUSE=1 for full backtrace
omegaconf.errors.ConfigKeyError: Key 'num_classes' is not in struct
    full_key: decoder.num_classes
    object_type=dict
```
so I decided to set the decode.num_classes to -1 but then I get this error:
```
    raise InstantiationException(msg) from e
hydra.errors.InstantiationException: Error in call to target 'nemo.collections.asr.modules.rnnt.RNNTDecoder':
TypeError("__init__() got an unexpected keyword argument 'num_classes'")
```