[Object] Implemented .as<T> for ObjectRef param, returns Optional<T>

[Lunderberg]

Prior to this PR, the ObjectRef::as<T>() method could be used for any T that inherits from tvm::Object, and would return a const T* if the class could be cast to the specified type, or nullptr otherwise. However, if the caller needed a ObjectRef, they would then need to call GetRef<MyObjRef> to convert from a const T*.

This PR extends ObjectRef::as<T> to operate on a T that inherits from tvm::ObjectRef as well. In this case, the return type is Optional<T>, returning either an instance of the specified subclass, or NullOpt if the object was not an instance of the specified subclass. Example usage of this new conversion, along with how it relates to existing functionality, is shown below.

// Unconditionally convert, throwing an exception if the object isn't
// of the specified type.  In contexts where the type of the object is
// unknown, this shouldn't be used.
PrimExpr expr = Downcast<PrimExpr>(obj);

// Protect the Downcast from throwing an exception using IsInstance.
// This avoids the error, but performs the type-checking twice.  In
// addition, it requires the caller to specify both the ObjectRef
// subclass and the Object subclass, even though these usually have a
// 1:1 correspondence.
if (obj->IsInstance<PrimExprNode>()) {
  PrimExpr expr = Downcast<PrimExpr>(obj);
}

// Perform both type-checking and downcasting with the ObjectRef::as()
// method, then use GetRef to convert to an ObjectRef.  This avoids
// double-checking the type, but still requires the caller to
if (const PrimExprNode* ptr = obj.as<PrimExprNode>()) {
  PrimExpr expr = GetRef<PrimExpr>(ptr);
}

// New method introduced by this PR.  The type-checking is only
// performed once, and the Object subclass is inferred from the
// ObjectRef subclass.
if (Optional<PrimExpr> opt = obj.as<PrimExpr>()) {
  PrimExpr expr = opt.value();
}

This PR is implemented as two commits. The first commit implements the new functionality, but makes no further changes. The second commit looks for cases where object_ref.as<TNode>() was immediately followed by GetRef<T>(), replacing with either object_ref.as<T>() or with Downcast<T>(object_ref).

[tvm-bot]

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[kparzysz-quic]

Looks good. Thanks!