Decouple LTC from TS Backend using Lazy IR Builder
Next stage of breaking up https://github.com/pytorch/pytorch/pull/74710
IR builder class introduced to decouple the explicit usage of `TsNode` in core lazy tensors.
Requires https://github.com/pytorch/pytorch/pull/75324 to be merged in first.
**Background**
- there are ~ 5 special ops used in lazy core but defined as :public {Backend}Node. (DeviceData, Expand, Scalar...)
- we currently require all nodes derive from {Backend}Node, so that backends can make this assumption safely
- it is hard to have shared 'IR classes' in core/ because they depend on 'Node'
**Motivation**
1. avoid copy-paste of "special" node classes for each backend
2. in general decouple and remove all dependencies that LTC has on the TS backend
**Summary of changes**
- new 'IRBuilder' interface that knows how to make 5 special ops
- move 'special' node classes to `ts_backend/`
- implement TSIRBuilder that makes the special TS Nodes
- new backend interface API to get the IRBuilder
- update core code to call the builder
CC: @wconstab @JackCaoG @henrytwo
Partially Fixes #74628
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75433
Approved by: https://github.com/wconstab
