tokenizer.add_tokens conflict with MBart's tokenizer
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
  
- `transformers` version: 3.1.0
- Platform: Linux-3.10.0-1127.el7.x86_64-x86_64-with-glibc2.10
- Python version: 3.8.3
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>


### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @TevenLeScao
 blenderbot: @mariamabarham
 Bart: @sshleifer
 Marian: @sshleifer
 T5: @patrickvonplaten
 Longformer/Reformer: @patrickvonplaten
 TransfoXL/XLNet: @TevenLeScao 
 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->
@patil-suraj @mfuntowicz 

## To reproduce

Steps to reproduce the behavior:

1. import MBartTokenizer
2. try `tokenizer.add_tokens('__something_new__')`
3. `tokenizer('__something_new__')`

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

`add_tokens()` should add new tokens after all existing tokens in the vocabulary, however, the `lang_code` will conflict with the added tokens.

## Current Workaround

```
new_token_to_id = {tok: vocab_size + i for i, tok in enumerate(new_tokens)}
self.tokenizer.fairseq_tokens_to_ids.update(new_token_to_id)
self.tokenizer.fairseq_ids_to_tokens = {v: k for k, v in self.tokenizer.fairseq_tokens_to_ids.items()}
self.tokenizer.unique_no_split_tokens.extend(new_tokens)
```

I love your workaround and would definitely be interested in a clean contribution that made in accessible to everyone.<|||||>@sshleifer Thanks! I've created a pull request #7353 with this workaround.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
