'MarianTokenizer' object has no attribute 'target_encoder'
### System Info

- `transformers` version: 4.19.0.dev0
- Platform: Linux-4.19.129-aufs-1-x86_64-with-debian-10.1
- Python version: 3.7.3
- Huggingface_hub version: 0.5.1
- PyTorch version (GPU?): 1.11.0+cu102 (False)
- Tensorflow version (GPU?): 2.8.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no


### Who can help?

@patil-suraj @SaulLu

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

model_name='Helsinki-NLP/opus-mt-en-he'
tokenizer = MarianTokenizer.from_pretrained(model_name)
tokenizer.get_tgt_vocab()


### Expected behavior

I expected to get the target vocab but instead, i got the error 
AttributeError: 'MarianTokenizer' object has no attribute 'target_encoder'
I need to find a way to separate vocab into the source and target vocabs instead of the current vocab which contains a mix of both languages.
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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