Upgrade exported ONNX IR version to 6 (#31025)
Summary:
Upgrade IR version from 4 to 6, below is change doc from ONNX. The upgrade should be backward compatible.
```
// IR VERSION 5 published on March 18, 2019
// - Add message TensorAnnotation.
// - Add quantization annotation in GraphProto to map tensor with its scale and zero point quantization parameters.
IR_VERSION_2019_3_18 = 0x0000000000000005;
// IR VERSION 6 published on Sep 19, 2019
// - Add support for sparse tensor constants stored in model.
// - Add message SparseTensorProto
// - Add sparse initializers
IR_VERSION = 0x0000000000000006;
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31025
Reviewed By: hl475
Differential Revision: D18935444
Pulled By: houseroad
fbshipit-source-id: 9ba47f9657fa1a668db291cf04af07d5e8d73c21
