[Unity] Unwrap tuples during CanonicalizeBindings (#15696)

Prior to this commit, `CanonicalizeBindings` would identify trivial
bindings of `var_y = var_x` and replace subsequent usage of `var_y`
with `var_x`, but bindings of the form `tuple_var = (var_x,); var_y =
var_x[0]` would not be canonicalized.

This commit updates the `CanonicalizeBindings` pass to identify
trivial bindings that occur across a `TupleGetItem` expression,
handling them as if they were direct assignments.

src/relax/transform/canonicalize_bindings.cc

@@ -36,6 +36,8 @@ class BindingCanonicalizer : public ExprMutator {
  public:
   BindingCanonicalizer() {}
 
+  using ExprMutator::VisitExpr_;
+
   Expr VisitExpr_(const VarNode* op) override {
     // remap first
     Var v = Downcast<Var>(ExprMutator::VisitExpr_(op));
@@ -54,6 +56,23 @@ class BindingCanonicalizer : public ExprMutator {
     return ExprMutator::VisitExpr_(LookupBinding(v).as<VarNode>());
   }
 
+  Expr VisitExpr_(const TupleGetItemNode* tuple_get_item) override {
+    if (auto tuple_var = tuple_get_item->tuple.as<Var>()) {
+      if (auto tuple_value = LookupBinding(tuple_var.value())) {
+        if (auto explicit_tuple = tuple_value.as<TupleNode>()) {
+          CHECK_GE(tuple_get_item->index, 0)
+              << "Tuple " << tuple_value << " is accessed at index " << tuple_get_item->index
+              << ", but negative indices are not supported in this context.";
+          CHECK_LT(tuple_get_item->index, explicit_tuple->fields.size())
+              << "Tuple " << tuple_value << " is accessed at index " << tuple_get_item->index
+              << ", but the tuple size is only " << explicit_tuple->fields.size();
+          return VisitExpr(explicit_tuple->fields[tuple_get_item->index]);
+        }
+      }
+    }
+    return ExprMutator::VisitExpr_(tuple_get_item);
+  }
+
   void VisitBinding_(const VarBindingNode* binding) override {
     // Unlike default visitor, we do not permit the checked type to change
     // if the new value's checked type is different (this preserves user annotations)

tests/python/relax/test_transform_canonicalize_bindings.py

@@ -254,5 +254,30 @@ def main(x: R.Tensor(("m", "n"))):
     assert_structural_equal(new_mod, Expected)
 
 
+def test_unwrap_tuple():
+    @tvm.script.ir_module
+    class Before:
+        @R.function
+        def main(x: R.Tensor, y: R.Tensor):
+            tuple_var = (x, y)
+            w = tuple_var[0]
+            q = tuple_var[1]
+            z = R.add(w, q)
+            return R.add(q, z)
+
+    @tvm.script.ir_module
+    class Expected:
+        @R.function
+        def main(x: R.Tensor, y: R.Tensor):
+            tuple_var = (x, y)
+            w = x
+            q = y
+            z = R.add(x, y)
+            return R.add(y, z)
+
+    after = relax.transform.CanonicalizeBindings()(Before)
+    assert_structural_equal(Expected, after)
+
+
 if __name__ == "__main__":
     tvm.testing.main()