onnx-export example notebook is failing for TF
hi, I'm using the latest 3.1.0 release.

When I run
```
from transformers.convert_graph_to_onnx import convert

# Tensorflow 
convert(framework="tf", model="bert-base-cased", output="onnx/bert-base-cased.onnx", opset=11)
```
as shown in https://github.com/huggingface/transformers/blob/master/notebooks/04-onnx-export.ipynb,

the following error occurs
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-bc5982e91176> in <module>
      7 
      8 # Tensorflow
----> 9 convert(framework="tf", model="bert-base-cased", output="onnx/bert-base-cased.onnx", opset=11)

/nix/store/w8xw8jng4dfjcqijfjw1sps8pim669kj-python3.7-transformers-3.1.0/lib/python3.7/site-packages/transformers/convert_graph_to_onnx.py in convert(framework, model, output, opset, tokenizer, use_external_format, pipeline_name)
    335     nlp = load_graph_from_args(pipeline_name, framework, model, tokenizer)
    336 
--> 337     if not output.parent.exists():
    338         print(f"Creating folder {output.parent}")
    339         makedirs(output.parent.as_posix())

AttributeError: 'str' object has no attribute 'parent'
```
The following should work:

```
from pathlib import Path
from transformers.convert_graph_to_onnx import convert

# Tensorflow 
convert(framework="tf", model="bert-base-cased", output=Path("onnx/bert-base-cased.onnx"), opset=11)
```<|||||>@subho406  thanks! I thought the error was about the model output since using string in output path had worked in the previous version.