ONNX Export for Fine-Tuned DistilBertForTokenClassification
# 🚀 Feature request

I'd like to export a fine-tuned DistilBertForTokenClassification model to ONNX. Right now the conversion script convert_graph_to_onnx.py looks like it takes a string like "--model bert-base-cased" but I'd like to pass in the model object that I've fine tuned on my data set. 

## Motivation

I've trained a custom token classifier that I want to run on the ONNX runtime. 

I figured out how to do it. Within python, you have to use `model.save_pretrained("path/to/output_dir")` where `model` is the fine-tuned `model = DistilBertForTokenClassification.from_pretrained('distilbert-base-uncased', num_labels=...)`

Then inside an empty directory, run
`python -m transformers.convert_graph_to_onnx --model path/to/output_dir --framework pt --tokenizer distilbert-base-uncased out.onnx`
