NMT combine embedding plus encoder/decoder into one neural module.
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This PR create encoder and decoder neural modules for `TransformerEmbedding` and `TransfomerEncoder` / `TransformerDecoder`.

This is making it so that the neural types will be the same across NeMo Transformer, HF BERT, and Megatron BERT encoders.

The idea is to have `EncoderModule` which accepts `input_ids`, ` encoder_mask`, and `num_tokentypes` and outputs `last_hidden_states`.

Then  `DecoderModule` will accept `input_ids`, `decoder_mask`, `encoder_mask` and `encoder_states` and outputs `last_hidden_states`.

This PR also cleans up the relevant configs as the configuration needed to be updated to accept the new model structure.