Update IR.md to describe training extension. (#2615)
Update IR.md to describe training extension. (#2615)

* Update IR.md to describe training extension.

* Update docs/IR.md

Co-Authored-By: Wei-Sheng Chin <259ee3e69901f7aab965f05802ec9db58fce7b2f@microsoft.com>

* Incorporate PR feedback suggestion

Co-authored-by: Wei-Sheng Chin <259ee3e69901f7aab965f05802ec9db58fce7b2f@microsoft.com>
Co-authored-by: Ke Zhang <368059018edb346f1fd2049a2d55b57da5b85a98@microsoft.com>

onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx