[Relax] Fix FuseOps: preserve symbolic shape dependencies from match_cast

[cchung100m]

Hi Committers,

This PR fixes issue #20195. Any suggestions would be appreciated if you are available.

Root Cause

FuseOps reorders bindings within a dataflow block by topologically sorting fusion groups according to OperatorFusor::group_deps_. That dependency map is built in CollectFuncBoundary, which only tracks a dependency when a binding's value references another binding's relax::Var directly.

A symbolic shape variable introduced by R.match_cast(..., R.Shape([s, ...])) is never bound by its own VarBindingNode, so it never gets its own graph node. If a later binding's resulting type uses that variable - e.g. a call_tir's out_ty=R.Tensor((s, ...)) - with no other relax::Var tying it back to the match_cast, no dependency edge is recorded. When an unrelated elementwise group happens to fuse, and its dependency chain transitively reaches that call_tir, TopoSortByGroupDep can schedule it before the match_cast that defines s, producing a module where s is used before it's defined (Symbolic Var ... is not defined.).

Solution

In OperatorFusor::CollectFuncBoundary (src/relax/transform/fuse_ops.cc):

• Track, per symbolic var, the group of the MatchCast binding that first defines it (DefinableTIRVarsInType(match_cast->ty)), in a new symbolic_var_def_group_ map.
• For every binding, additionally scan the symbolic vars referenced by its resulting type (TIRVarsInType(GetType(binding->var))) and add a group dependency on the recorded definer, same as for ordinary relax::Var dependencies (cyclic-dependency check reused via a shared AddGroupDep helper).

[cchung100m]

cc @tlopex @mshr-h