MarianMT - ONNX only accepts fixed input despite setting dynamic axes
## 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-5.8.0-43-generic-x86_64-with-glibc2.29 (Ubuntu 20.04.2)
- Python version: 3.8.3
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes (No too, error persists in both cases)
- Using distributed or parallel set-up in script?: No

### Who can help
marian: @patrickvonplaten, @patil-suraj 

## Information

Model I am using (Bert, XLNet ...): Helsinki-NLP/opus-mt-en-hi (bug persists for other languages too)

The problem arises when using:
* [ ] my own modified scripts: (give details below)
 I slightly modified the  `convert_graph_to_onnx.py` script with the code snippet given below(call to export is exactly the same). Apparently torch.triu() is not supported for onnx conversion, so following prior issues in PyTorch [#32968](https://github.com/pytorch/pytorch/issues/32968) I modified the script, resulting in successful onnx conversion of models.


        torch_triu = torch.triu

        def triu_onnx(x, diagonal=0):
            l = x.shape[0]
            arange = torch.arange(l, device=x.device)
            mask = arange.expand(l, l)
            arange = arange.unsqueeze(-1)
            if diagonal:
                arange = arange + diagonal
            mask = mask >= arange
            return x.masked_fill(mask == 0, 0)

        torch.triu = triu_onnx
        export(
            nlp.model,
            model_args,
            f=output.as_posix(),
            input_names=ordered_input_names,
            output_names=output_names,
            dynamic_axes=dynamic_axes,
            do_constant_folding=True,
            use_external_data_format=use_external_format,
            enable_onnx_checker=True,
            opset_version=opset,
        )
        torch.triu = torch_triu
The tasks I am working on is:
* Simple Machine Translation

## To reproduce

Steps to reproduce the behavior:

1. ```python convert_graph_to_onnx.py --framework pt --model Helsinki-NLP/opus-mt-en-hi onnx-models/opus-mt-en-hi.onnx```
2.  
```
from transformers import AutoTokenizer
from onnxruntime import ExecutionMode, InferenceSession, SessionOptions
import numpy as np 

tok_name = 'Helsinki-NLP/opus-mt-en-hi'
model_name = 'onnx-models/opus-mt-en-hi.onnx'

tokenizer = AutoTokenizer.from_pretrained(tok_name)
options = SessionOptions()
options.intra_op_num_threads = 1
options.execution_mode = ExecutionMode.ORT_SEQUENTIAL
session = InferenceSession(model_name, options)

tokens = tokenizer.encode_plus('Testing onnx conversion through a sample input for machine translation.')
tokens = {name: np.atleast_2d(value) for name, value in tokens.items()}
op = session.run(None, tokens)
```
3. Stack Trace

```
---------------------------------------------------------------------------
RuntimeException                          Traceback (most recent call last)
<ipython-input-96-d76889a45083> in <module>
     14 tokens = tokenizer.encode_plus('Testing onnx conversion through a sample input for machine translation.')
     15 tokens = {name: np.atleast_2d(value) for name, value in tokens.items()}
---> 16 op = session.run(None, tokens)

~/.pyenv/versions/3.8.3/lib/python3.8/site-packages/onnxruntime/capi/onnxruntime_inference_collection.py in run(self, output_names, input_feed, run_options)
    122             output_names = [output.name for output in self._outputs_meta]
    123         try:
--> 124             return self._sess.run(output_names, input_feed, run_options)
    125         except C.EPFail as err:
    126             if self._enable_fallback:
```
```
RuntimeException: [ONNXRuntimeError] : 6 : RUNTIME_EXCEPTION : Non-zero status code returned while running Reshape node. Name:'Reshape_62' Status Message: /onnxruntime_src/onnxruntime/core/providers/cpu/tensor/reshape_helper.h:42 onnxruntime::ReshapeHelper::ReshapeHelper(const onnxruntime::TensorShape&, std::vector<long int>&) gsl::narrow_cast<int64_t>(input_shape.Size()) == size was false. The input tensor cannot be reshaped to the requested shape. Input shape:{1,13}, requested shape:{5}
```
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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
Ideally the input should have smoothly passed through the onnx converted model during inference, but it doesn't.

Possible useful information
```
Dynamic Axes:
 {'input_ids': {0: 'batch', 1: 'sequence'},
 'attention_mask': {0: 'batch', 1: 'sequence'},
 'output_0': {0: 'batch', 1: 'sequence'},
 'output_1': {0: 'batch', 1: 'sequence'}}
```
```
Generated inputs order: ['input_ids', 'attention_mask']
```
**My lead is that** - 
Exporting is not taking into account the dynamic axes for some reason, when any Marian Mt model is being used.
The error also notes - requested shape size - 5, which is the sequence length of the dummy input (line 196 in `convert_graph_to_onnx.py` while converting to onnx.
Notably, passing an input with sequence length 5 works perfectly fine.
Moreover, this script works perfectly for standard models like distilbert for both model conversion, and inference. So it's surely some model-specific problem.

Will be really helpful to get a fix for this, especially since there are numerous Marian mt models so it can have a larger impact!
pinging @mfuntowicz, our `onnx` expert.<|||||>Any lead on the solution? @mfuntowicz, @patrickvonplaten, @patil-suraj <|||||>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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