Add ONNX support for MarianMT models
# What does this PR do?

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This PR adds support to export MarianMT models in the ONNX format. The underlying logic builds on the awesome refactor / feature enhancement that @michaelbenayoun has implemented in #14358 & #14700 - ~we should rebase this branch on `master` once that PR is merged to simplify the diff in this PR.~ (Done)

Currently, this PR supports ONNX exports for the following "tasks" (i.e. uses):

* `default`, `default-with-past` => equivalent to exporting a pretrained `MarianModel`
* `seq2seq-lm`, `seq2seq-lm-with-past` => equivalent to exporting a pretrained `MarianMTModel`
* `causal-lm`, `causal-lm-with-past`=> equivalent to exporting a pretrained `MarianForCausalLM`

Note that in each case, the end user will have to implement their own `generate()` method with the ONNX model - see [this BART example](https://github.com/huggingface/transformers/tree/master/examples/onnx/pytorch/summarization) for what's involved.

I've also checked locally that the "slow" tests pass with:

```
RUN_SLOW=1 pytest tests/test_onnx_v2.py -k "marian" -rp
```

## Usage

Here's a quick example to show how this works:

```python
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM
from transformers.models.marian import MarianOnnxConfig

model_ckpt = "Helsinki-NLP/opus-mt-en-de"
tokenizer = AutoTokenizer.from_pretrained(model_ckpt)
ref_model = AutoModelForSeq2SeqLM.from_pretrained(model_ckpt)
# Export model
feature = "seq2seq-lm"
onnx_path = f"onnx/{model_ckpt}-{feature}/"
# Run this from a Jupyter notebook
!python -m transformers.onnx --model={model_ckpt} --atol=1e-4 --feature={feature} {onnx_path}
# Test export with inputs
batch_size = 4
encoder_inputs = tokenizer(
    ["Studies have been shown that owning a dog is good for you"] * batch_size,
    return_tensors="np",
)
decoder_inputs = tokenizer(
    ["Studien haben gezeigt dass es hilfreich ist einen Hund zu besitzen"]
    * batch_size,
    return_tensors="np",
)
all_inputs = {
    "input_ids": encoder_inputs["input_ids"],
    "attention_mask": encoder_inputs["attention_mask"],
    "decoder_input_ids": decoder_inputs["input_ids"],
    "decoder_attention_mask": decoder_inputs["attention_mask"],
}
# Generate ONNX outputs
ort_session = ort.InferenceSession(f"{onnx_path}model.onnx")
onnx_config = MarianOnnxConfig(ref_model.config, task=feature)
onnx_named_outputs = list(onnx_config.outputs.keys())
onnx_outputs = ort_session.run(onnx_named_outputs, all_inputs)
```

**TODO**

- [x] Extend support for language modelling head
- [x] Investigate range of numerical tolerance between raw and ONNX models for a range of checkpoints
- [x] Ensure that ONNX models are compatible with ONNX Runtime
- [x] Verify whether past key values are supported

Closes #13823, #13854 




## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

 -->

There seems to be some sort of race condition happening in `run_tests_torch`:

```
_____________________________ ERROR collecting gw1 _____________________________
Different tests were collected between gw0 and gw1. The difference is:
--- gw0

+++ gw1
```

[This issue](https://github.com/pytest-dev/pytest-xdist/issues/366) has similar problems - perhaps a solution lies there.<|||||>Feel free to merge once you have taken care of the docs and the `# Copied from` statements :)<|||||>Thanks for the reviews @LysandreJik and @michaelbenayoun 🙏 !

I've fixed the docs by rebasing on `master` and added the `# Copied from:` snippets to the functions (I did not know about that trick!)

Will merge once all the test pass :)<|||||>The outputs decoding cannot get the correct result. How do you get the translation result