Export onnx model support

[qibaoyuan]

Motivation and Context

I have use org.bytedeco.javacpp-presets and deeplearning4j in my JAVA application to do Deep learning task in production env, and it can use onnx as a pre-trained model. So, I make this PR to copy the model as a .onnx file after it was converted from pytorch-type model.I make a "export_onnx_path" para, in this PR I split "--output-model-path" to "--output-caffe2-path" and "--output-onnx-path" in "export" command.

How Has This Been Tested

my test code under JAVA applciation in javacpp-presets is as belows:

onnx.OpSchemaVector allSchemas = onnx.OpSchemaRegistry.get_all_schemas();
System.out.println(allSchemas.size());

byte[] bytes = 
 Files.readAllBytes(Paths.get("/Users/qibaoyuan/Documents/source/project/pycharm/pytext/minlpdemo/ner/model.pt.prd.2.onnx"));

onnx.ModelProto model = new onnx.ModelProto();
ParseProtoFromBytes(model, new BytePointer(bytes), bytes.length);

check_model(model);

InferShapes(model);

onnx.StringVector passes = new onnx.StringVector("eliminate_nop_transpose", "eliminate_nop_pad", "fuse_consecutive_transposes", "fuse_transpose_into_gemm");
Optimize(model, passes);

check_model(model);

ConvertVersion(model, 8);

System.out.println(model.graph().input_size());

or in deeplearning4j:

    public static OnnxProto3.GraphProto loadModel(final String file) {
        try (InputStream is = new ClassPathResource(file).getInputStream()) {
            OnnxProto3.GraphProto graphProto = OnnxProto3.GraphProto.parseFrom(is);
            log.debug("load model from onnx:[{}]", graphProto);
            return graphProto;
        } catch (Exception ex) {
            log.error("error:[{}]", ExceptionUtils.getStackTrace(ex));
        }
        return null;

    }

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[qibaoyuan]

@seayoung1112 I have fixed the requested changes, please check.

[facebook-github-bot]

@seayoung1112 has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.