XLNet ONNX model giving error: "Attempting to broadcast an axis by a dimension other than 1"
## Environment info
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- `transformers` version: 3.5.1
- Platform: Linux-4.14.193-113.317.amzn1.x86_64-x86_64-with-glibc2.10
- Python version: 3.7.6
- PyTorch version (GPU?): 1.7.0 (True)
- Tensorflow version (GPU?): 2.3.1 (True)
- Using GPU in script?: False
- Using distributed or parallel set-up in script?: False

### Who can help
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@TevenLeScao @mfuntowicz @patil-suraj 

## Information

Model I am using (Bert, XLNet ...): XLNet

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Trained HuggingFace Transformers model XLNetForSequenceClassification on custom dataset with PyTorch backend.
2. Used provided `convert_graph_to_onnx.py` script to convert model (from saved checkpoint) to ONNX format.
3. Loaded the model with ONNXRuntime
4. When feeding in int64 numpy arrays `input_ids` and `attention_masks`, the model returns the following error except when both inputs have shape (x, 1) or (x, 6). There is nothing in the configuration of the model or the structure of the training data from my end that would require shape (x, 1) or (x, 6).

```
RuntimeException: [ONNXRuntimeError] : 6 : RUNTIME_EXCEPTION : Non-zero status code returned while running Add node. Name:'Add_26' Status Message: /onnxruntime_src/onnxruntime/core/providers/cpu/math/element_wise_ops.h:361 void onnxruntime::BroadcastIterator::Init(int64_t, int64_t) axis == 1 || axis == largest was false. Attempting to broadcast an axis by a dimension other than 1. 5 by 6
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
The expected behavior is for the model to return predictions successfully (i.e. probabilities for all classes).
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