[QoL][Relax] Use SeqExpr in IR types when SeqExpr is required

[Lunderberg]

The Relax IR requires the FunctionNode::body, IfNode::true_branch, and IfNode::false_branch to be instances of relax::SeqExpr. If these Relax requirements are violated, correctly-implemented transformations may raise an exception
(e.g. from Downcast in Downcast<SeqExpr>(func->body)->blocks), or even segfault (e.g. when .as returns a nullptr in func->body.as<SeqExprNode>()->blocks). Debugging these failure modes is also difficult, as even the TVMScript printer relies on the body of the function being a SeqExprNode.

This commit updates the C++ type of FunctionNode::body, IfNode::true_branch, and IfNode::false_branch to be relax::SeqExpr instead of relax::Expr. This does not impact any well-formed Relax IR, and allows this type of ill-formed Relax IR type to be checked at compile-time. A large number of checks applied during TVM runtime can now be removed, as they duplicate the new compile-time check.

To maintain backwards compatibility, this commit adds a new constructor to relax::SeqExpr, which accepts a single Expr body argument. This constructor provides either an additional reference to the same underlying relax::SeqExprNode, if body already contains a relax::SeqExprNode, and otherwise wraps the body in a relax::SeqExpr. For implementations that previously produced well-formed Relax IR, this change has no effect. For implementations that previously produced ill-formed Relax IR, this change results in the equivalent well-formed Relax IR.

Alternate implementations considered:

• Perform the backwards-compatibility wrapping within the relax::Function and relax::If constructors. While this would provide the intended conversion when these constructors are used, Relax transforms make frequent use of copy-on-write (e.g. func.CopyOnWrite()->body = new_body), which does not use the constructor. Maintaining backwards compatibility for this usage requires the implicit conversion constructor that was chosen for this PR.

• Remove the Relax IR requirement for these expressions to be SeqExpr. While this would make Relax more internally consistent, such a change would break backwards compatibility that relies on SeqExpr being present. While the callsites within TVM could be updated to resolve this breakage, callsites outside of TVM (e.g. MLC-LLM) could not. Exposing the special case within the C++ type, as done in this PR, maintains backwards compatibility.

[Lunderberg]

This PR is currently marked as a draft, to allow time for community discussion. A forum thread (link) for discussion has also been started for this change.

[quic-sanirudh]

Thanks a lot for this. Looks good to me apart from a minor comment about possibly modifying IfNode in place to improve git history maintenance to just highlight the changed lines instead of the whole class.