Cannot unset both `source_processor` and `target_processor` for NMT model inference.
If I omit both parameters `--target_lang` and `--source_lang` when running script `nmt_transformer_infer.py`, then both `target_processor` and `source_processor` attributes of model `MTEncDecModel` are set as during the model training.

This behaviour appears because 
1) `source_processor` and `target_processor` are set while [restoring from checkpoint](https://github.com/NVIDIA/NeMo/blob/a089791db1d4be51da2cd7cdec2eb822ca6b588c/examples/nlp/machine_translation/nmt_transformer_infer.py#L154),
2) `source_processor` and `target_processor` are [reset](https://github.com/NVIDIA/NeMo/blob/a089791db1d4be51da2cd7cdec2eb822ca6b588c/nemo/collections/nlp/models/machine_translation/mt_enc_dec_model.py#L858) only if at least one of parameters `--source_lang`, `--target_lang` are provided.

To produce the problem you need
1) find `.nemo` file for `MTEncDecModel` model which was trained with source or target processor,
2) add some debug logging [here](https://github.com/NVIDIA/NeMo/blob/a089791db1d4be51da2cd7cdec2eb822ca6b588c/nemo/collections/nlp/models/machine_translation/mt_enc_dec_model.py#L763) to see if processing is performed,
3) launch script `nmt_transformer_infer.py` without parameters `--source_lang` and `--target_lang`.