[executorch][native] Add native runtime Program reader + DOT visualizer

[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

First C++ runtime components for the native backend, under
backends/native/runtime/:

• ptn::Program (Program.h / Program.cpp): a standalone reader that parses
  and verifies a serialized native graph (*.ptg). Program::load() copies
  the bytes, checks the NPTG FlatBuffer file identifier, runs the FlatBuffer
  verifier, and exposes a zero-copy view of the root; it throws
  std::runtime_error on failure. No ExecuTorch or torch dependency
  (flatbuffers only); exceptions allowed. Our sources build as C++20 -- the
  --cpp-std c++11 in the genrule sets the feature level of the generated
  accessors, not ours (flatc accepts only c++0x / c++11 / c++17).
• ptn::to_dot(const Program&) (utils/ToDot.h / utils/ToDot.cpp): renders
  the program to Graphviz DOT text -- methods as clusters, dataflow / def-use
  as labeled edges, tensor metadata, constants, alias / mutation facts, quant,
  and HOP subgraphs. Pure string builder, no I/O. Doubles as the reference walk
  of native_backend::Program.

The renderer is a free function in its own :to_dot target rather than a
Program member, so a link that only reads programs does not carry it:
:runtime depends on the generated schema alone, while :to_dot pulls in
graph:string_format.

Also wires a --dot flag into the ptg_inspector tool: load + verify via the
reader, then print the DOT to stdout.

The schema header is generated from serialization/native_graph.fbs via a
flatc genrule (:native_graph_schema). Mirrored into both the fbcode/ and
xplat/ trees to match the native backend layout.

Authored with Claude Code.

Differential Revision: D114396768

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22274

• 📄 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 bad0d64 with merge base a5f15b5 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/native/runtime/utils/ToDot.cpp:

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.