[Unity][TVMScript] Update call_packed semantics to support empty sinfo_args

[Hzfengsy]

In low-level Relax (after the pass CallTIRewrite), the call_packed nodes do not always have explicit sinfo_args. This PR extends the parser to support this case.

[Hzfengsy]

cc @junrushao @cyx-6

[slyubomirsky]

What is the resulting StructInfo if sinfo_args is empty?

[Hzfengsy]

It should be the R.Object, which only happens in the low-level stage I think

[Lunderberg]

It should be the R.Object, which only happens in the low-level stage I think

At the low-level relax, does this correspond to a void return type?

[Hzfengsy]

At the low-level relax, does this correspond to a void return type?

You are right, it's void. But for now it is printed as an Object. Here is a real-world example:

_5_1: R.Object = R.call_packed("vm.builtin.paged_attention_kv_cache_attention", paged_kv_cache, R.prim_value(0), alloc15, alloc16, alloc17, alloc18)

[Lunderberg]

Ah, thank you. I had interpreted not having "explicit sinfo_args" as having a null pointer for the Array<StructInfo> sinfo_args, and wanted to check if that would instead indicate a bug in CallTIRRewrite. Instead, CallTIRRewrite produces a well-defined, but empty, sinfo_args. The mismatch is in the TVMScript printer, which suppresses printing of empty sinfo_args, and the TVMScript parser, which currently requires sinfo_args even if empty.

Thank you for clearing up my confusion.