descript: roundtrip descript/scheduler

[qaco]

Motivation

We know how to build a scheduler from descript, but we also want to build descript from a scheduler.

Description

1. The flat representation is replaced by a tree structure (where children are created by splits).
2. The parsing, the data structure and the interpretation are splitted in different files.
3. The data structure (LoopNestNode) now embeds a pretty printer.
4. A Scheduler can now build a LoopNest.
5. The declarative DSL now exposes buffer_at
6. The declarative DSL now exposes pack_at

[guillon]

Is there a possibility to make the LoopNest/LoopChild being even further redesigned into a single typical LoopNestTree structure, for instance with child/parent relationship?

[qaco]

Is there a possibility to make the LoopNest/LoopChild being even further redesigned into a single typical LoopNestTree structure, for instance with child/parent relationship?

Yeay done. With a generic tree structure

[qaco]

Orthogonally, here appears my splitting misconception spotted by @yadej ; the 'rematerialization' of j inside a branch is needed by the MLIR backend, but should not be exposed in descript or outside of the backend.