This PR lets Relax expressions directly take `PrimExpr` values without
requiring the explicit `PrimValue` wrapper, continuing the Relax IR
unification work by removing Relax-specific leaf/base expression layers.
Summary:
- Remove `LeafExpr` / `LeafExprNode` and use direct expression-node
checks where needed.
- Converge Relax expression typing onto the shared IR `Expr` base.
- Remove the `PrimValue` node wrapper while keeping `relax.prim_value` /
`R.prim_value` as conversion helpers that return existing `PrimExpr`
values unchanged.
- Register direct `PrimExpr` handling through exact concrete node
dispatch, aligned with the `tirx` expression visitor list and excluding
arith iter-map intermediate nodes.
- Inline the private Python primitive conversion helper into public
`relax.prim_value`.
- Handle direct `PrimExpr` values in frontend scalar paths without
assuming a `.value` field on non-immediate expressions.
Tianqi Chen
120812e9ac