Refactor with std::variant (on device training) (#12383)
* use std::variant for synthetic data storage.
* use std::variant to replace TypedCheckpointProperty
* Remvoe shared ptr for checkpoint property
* fix tests
* refine std::variant usage a bit
* remove CheckpointProperty data abstraction
* use InlinedVector and InlinedHashMap if possible
* fix comments
* fix build and test
* fix some comments
* use gsl::span
* fix tests
* refine based on comments
* fix win build
* fix build
