* There is a merge conflict when merge PR #2914
* Develop and PR #2914 both add `DDim::size` method, but did not
  triger git merge conflict while merge.

* Also fix unit test

* Implement InferShape and register them, give a stub Kernel method
  by LOG(INFO)

1. Refactor DDim's product() to make it more efficiently.

2. Add slice_ddim().