Inference with onnx (Image matting)

[manthan2305]

1. Added a modified version of the modnet.py file without pred_semantic and pred_details as both returns None when inference=True, which will make it easy to convert onnx model.

2. export_modnet.py will take checkpoint file as an input and generate onnx model with dynamic input and output shape.

3. Inference code is also provided for image matting.

4. Try this colab demo

5. Download modnet.onnx

[ZHKKKe]

@manthan3C273
I have processed this pull request.
The code has been reorganized under the folder onnx.
Big thanks.

[manthan2305]

Thank you, @ZHKKKe
Your changes made it easy to access. And I would love to contribute more.!

[ZHKKKe]

@manthan3C273 We are open for any other contributions!