[PyTorch Bart] Split Bart into different models
# What does this PR do?

This PR splits all Bart-like models into their own respective classes for PyTorch models only. This is more in line with the general philosophy of the library to have self-contained model files.

As discussed with @jplu, the TF models will be separated in a future PR as there are still some issues and improvements (TF serving) blocking the separation - see https://github.com/huggingface/transformers/issues/9313. 

In short, after this PR all those "model-specific" config parameters are removed from all Bart-like configs:

- `extra_pos_embeddings`
- `normalize_embedding`
- `add_final_layer_norm`
- `normalize_before`
- `do_blenderbot_90_layernorm`
- `static_position_embeddings`
- `add_bias_logits`
- `force_bos_token_to_be_generated` (this one has to be kept for Bart though)

and each "bart" model (Pegasus, Bart, MBart, Marian, Blenderbot, BlenderbotSmall) will get its own `modeling_....py` file.

At the moment the models have the following configurations:

| | `extra_pos_embeddings` |  `normalize_before`   | `add_final_layer_norm`  | `do_blenderbot_90_layernorm` | `normalize_embedding` | `static_position_embeddings` | `add_bias_logits` | `force_bos_token_to_be_generated` |
|:---|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
| `bart` |  2 | ❌  | ❌   | ❌  |  ✔️  | ❌  | ❌ | ✔️ |
| `mbart` | 2  | ✔️ |  ✔️   | ❌  |  ✔️  | ❌  |  ❌  | ❌  |    
| `marian` | ❌ | ❌   |  ❌  | ❌  | ❌   | ✔️  | ❌   | ❌  |    
| `pegasus` | ❌  |  ✔️  |  ✔️  | ❌  |  ❌  | ✔️  |  ❌  | ❌  |    
| `blenderbot90M (BlenderbotSmall)` | 0  | ❌  |  ❌  | ✔️   | ✔️   | ❌  |  ❌  |  ❌ |    
| `blenderbot3B + rest (Blenderbot)` | 0  |  ✔️  | ✔️   | ✔️  | ❌   | ❌  |  ❌  |  ❌ |    

We can see that `add_bias_logits` is actually never used, so I think the best option is to just delete the functionality. Also, one can see that no two models have the exact same usage of the above params, so we'll make 6 different modeling_....py files.

## Resulting Improvements:

- The model files are much more readable and should be much easier to navigate for the user. No difficult config parameters anymore where the user doesn't know what to set anyways, such as  `normalize_before`. 

- All irrelevant Bart-like features for other models are removed. Those features are a) never mentioned in the paper, b) don't make any sense since the model wasn't trained with those features, so that the usage of those features leads to non-sense outputs. *E.g.* Marian was never supposed to be a "mask-filling" model, yet it has "mask-filling" functionality, when doing:

```python
marian = MarianMTModel.from_pretrained(...)
marian(input_ids)  # no decoder_input_ids for mask filling like tasks such as in Bart
# => output makes 0 sense
```

The big gain here is that users are better guided on how to use the model and wonder less about whether the model is used correctly & whether there is a bug in the model.

- Docstrings are improved with more Model-specific examples and fewer comparisons to Bart. *E.g.* Pegasus, Marian, and Blenderbot never really mention BART in their paper and have no direct relation to BART IMO => these models should not be compared to BART in the docs -> it's confusing for the user

- Some small improvements, memory is slightly improved for beam search and gradient checkpointing is added.
- All previous tests are copied + some additional tests are added for each model

## Possible drawback

- The drawback as expected is code duplication. This is remedied to some extent by using the # Copied from ... safety features
- Some breaking changes as explained further below
- Models might now diverge easier in the future which could make it harder to have the same API for training. This is however also prevented by some function signature tests that are already implemented.

## Breaking changes

🚨🚨 **Important: We cannot keep 100% backward compatibility here or the PR won't make much sense** 🚨🚨
- Since all models were packed into a single model file a lot of different model design are at the moment possible. E.g.
Pegasus was only ever used with Sinusoidal position embeddings (as mentioned in the paper) but since it's merged into `modeling_bart.py`, one could theoretically use Pegasus with Learned position embeddings. This is not done in any config on the model hub however and will not be possible anymore after the PR. Also, Marian's model design has never normalized the word embeddings, but it could be possible with the current design. But again no config in the model hub does that, so this will also not be possible anymore after the PR. **In short: All model designs that were never foreseen in the original model and that are never used on the model hub at the moment won't be allowed anymore after the PR**. 
If we would not make this change, it would mean that we would have to keep all those `normalize_before` configs, which in return would mean that the modeling code of all Bart-like models would be the same again.

- Blenderbot needs to be divided into two models IMO. Blenderbot 90M not only has a very different architecture (see table above), but also uses a different tokenizer. I created a new `BlenderbotSmallModel` class. Thus I need to update one Blenderbot config online, changing it's class. This means that from this PR onward the following is not supported anymore:

```python
from transformers import BlenderbotForConditionalGeneration

model = BlenderbotForConditionalGeneration.from_pretrained("facebook/blenderbot-90M")
# => this is a wrong model. It should be
model = BlenderbotSmallForConditionalGeneration.from_pretrained("facebook/blenderbot-90M")
```

That's a big breaking change, but I don't see another way. If we keep the small blenderbot in the "normal" blenderbot, we have to keep the config params `normalize_before` which I really don't want to do.... I think the best option here is to add a warning (or even an error) by overwriting `from_pretrained(...)` in `BlenderbotForConditionalGeneration` so that 
```python
model = BlenderbotForConditionalGeneration.from_pretrained("facebook/blenderbot-90M")
```
will throw an error or give a warning. There are no fine-tuning blenderbot models on the hub and this is the only config. I think it's the right approach to separate the model here

- Barthez has essentially a `mbart` architecture, but has `bart` defined as its `model_type` in the configs. Here I'd also like to change the configs online to make sure the correct model is loaded when using `AutoModelForSeq2SeqLM`. I should also contact the author here.

- Bart allowed to automatically create `decoder_input_ids` by shifting the `input_ids` to the right. Thus, in Bart one can do the following:

```python
bart = BartForConditionalGeneration(...)
bart(input_ids) # not that no decoder_input_ids are passed here
```

This is a very special case and should only be used for Bart-like denoising pre-training or mask-filling. The only models that were trained in this fashion and thus can do mask-filling are Bart and MBart. All other models cannot do mask-filling so that `decoder_input_ids` should never be created from shifting `input_ids`. => this feature is removed therefore from Pegasus, Marian, Blenderbot, and BlenderbotSmall

Those are all breaking changes. Blenderbot is the big one, the other one should be fine. To be sure, I wrote some scripts that verify that no model on the model hub that contains one of the keywords `bart`, `mbart`, `pegasus`, `blenderbot`, `opus-mt`, `barthez` has incorrect/unexpected parameter settings after the PR.

## TODO:

- [x] Create Bart model file & pass all tests
- [x] Create MBart model file & pass all tests
- [x] Greate Pegasus model file & pass all tests
- [x] Create Marian model file & pass all tests
- [x] Create Blenderbot model file & pass all tests
- [x] Create BlenderbotSmall model file & pass all tests
- [x] Clean PR (delete all helper files)
- [x] Clean docs
- [x] Add #Copied From statements
- [x] To a very in-detail review of own PR to make sure no hidden bugs were introduced.
- [x] Correct configs of barthez online to be of type `mbart` instead of `bart`.
- [x] Correct config of https://huggingface.co/facebook/blenderbot-90M online.

## Future TODO:
- [ ] Communitace about this PR on the forum
- [ ] Add Copied From statements to seq2seq bart model templates
- [ ] Add Copied From statements to LED

One important comment I forgot to add to my review: I don't think we should adapt the `research_project` to the new structure as it has been pinned to an earlier version of transformers (3.5.1). So apart from the duplicate file deleted, the other changes should be reverted IMO.