[executorch][native] Add Argument tagged union to the in-memory IR

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Add deserializer bridge (Program -> Method) #22105
• [executorch][native] Add Method + ValueRole to the in-memory IR #22104
• [executorch][native] Add Graph index arena to the in-memory IR #22103
• [executorch][native] Add Node to the in-memory IR #22102
• -> [executorch][native] Add Argument tagged union to the in-memory IR #22101
• [executorch][native] Add Value + index-arena ids to the in-memory IR #22100
• [executorch][native] Add Scalar value type to the in-memory IR #22099
• [executorch][native] Add TensorMeta + ScalarType in-memory IR types #22098
• [executorch][native] Add native runtime Program reader + Graphviz visualizer #22097
• [executorch][native] Add native_executor test harness #22096

Adds ptn::Argument, the in-memory form of an fx node argument, mirroring the
schema ArgumentValue union but with in-graph references resolved to ValueRef
arena indices instead of SSA-name strings (the deserializer will do the name ->
ref resolution).

Argument is a hand-rolled real tagged union: an ArgKind tag over a C union
of the 13 payload structs (TensorArg, IntArg, TensorListArg, GraphArg,
...). This matches the value layer's existing style (Scalar, Value) and
keeps storage compact (size = largest payload + tag) rather than pulling in
std::variant. Because several payloads are non-trivial (std::string,
std::vector), lifetime is managed by hand via destroy() /
construct_from(const&) / construct_from(&&); copy-assign builds a temporary
then steals it through the noexcept move-assign for a strong exception
guarantee. Payloads are read via kind() + throwing as_*() accessors, the
same idiom as Value.

Symbolic scalar args carry both a literal value and a ref (valid ref =>
symbolic, e.g. a sym_size result). OptionalTensorListArg collapses the
schema's parallel has_value[] into a kInvalid sentinel per element.
GraphArg holds a GraphRef (index into a subgraph arena that lands with
Graph/Model), not an owned subgraph, so Argument needs no Graph
dependency, forward declaration, or incomplete-type destructor. NamedArgument
wraps an Argument with its op-schema parameter name and an in-place mutated
flag.

Also adds using GraphRef = int32_t; to Ids.h for the GraphArg handle.

Pure std only (no ExecuTorch, no flatbuffers), consistent with the rest of the
standalone ptn runtime.

Differential Revision: D114396764

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22101

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ No Failures

As of commit 71e74aa with merge base 9a2d135 ():
💚 Looks good so far! There are no failures yet. 💚

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.