[ONNX2TRT] Fix Bug of Exporting ONNX Model & Supporting 320x320 Image Size.

[kevhsu-k]

Issue #, if available:

Description of changes:

• Fixing exporting onnx model error. (Error occurs when building trt engine)
• Support image size 320x320.
• Move model to S3 and provide a download script.
• Add onnx model with image size 320x320
• Add image size parameters inside graph.json. Let usre choose which image size and model to use. Currently support 320 and 640.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kevhsu-k]

@SuryaKari I haved added more description on exporting the pt model to onnx. And also provide a way to test compiling engine and inference using engine over an GPU EC2 instance. Please help review. Thanks.