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

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Add Method + ValueRole to the in-memory IR #22281
• [executorch][native] Add Graph index arena to the in-memory IR #22280
• [executorch][native] Add Node to the in-memory IR #22279
• -> [executorch][native] Add Argument tagged union to the in-memory IR #22278
• [executorch][native] Add Value + index-arena ids to the in-memory IR #22277
• [executorch][native] Add Scalar value type to the in-memory IR #22276
• [executorch][native] Add TensorMeta + ScalarType in-memory IR types #22275
• [executorch][native] Add native runtime Program reader + DOT visualizer #22274

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

Argument is a tagged union over the 13 payload structs (TensorArg, IntArg,
TensorListArg, GraphArg, ...), backed by a std::variant whose alternatives
are declared in ArgKind order so kind() is the variant's index. A
static_assert pins each kind to its alternative and checks the two lists are
the same length, so they cannot drift apart -- and the variant keeps the
non-trivial payloads (std::string, std::vector) correct without hand-written
lifetime management. Payloads are read via kind() + throwing as_*()
accessors, the same idiom as Value.

Symbolic scalar args carry both a literal value and an id (valid id =>
symbolic, e.g. a sym_size result). OptionalTensorListArg collapses the
schema's parallel has_value[] into a kInvalid sentinel per element.
GraphArg holds a GraphId (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 GraphId = int32_t; to Ids.h for the GraphArg handle, and
extends the IR printer (utils/Print.{h,cpp}) with to_string(const Argument&).

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/22278

• 📄 Preview Python docs built from this PR

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

❌ 1 New Failure, 1 Unrelated Failure

As of commit 73ba529 with merge base a5f15b5 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  adb: device offline

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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.

[digantdesai]

Review automatically exported from Phabricator review in Meta.