[executorch][native] Add Method + ValueRole 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::Method, the stateful wrapper around one top-level Graph, and the
per-value ValueRole classification it drives. Graph is a pure function body;
Method binds its placeholder and output values to the things outside the pure
dataflow — external constant data, non-persistent buffers, and the per-output
classification that separates a real result from a state writeback. It mirrors
the schema Method but resolves every in-graph reference to a ValueRef.

Method lives at runtime/ (peer to the Program reader), above the graph/
arena package, since it is a method-level rather than a graph-level concept.

New types:

• ValueRole (on Value, in graph/Value.h): Intermediate / UserInput /
  Parameter / Buffer / ConstantTensor. Value also gains a data_key string.
  Both are stamped by the deserializer and give O(1) per-value answers to "how
  is this value's storage owned / sourced" (the memory planner and runtime
  binder are the eventual consumers).

• DataBinding (schema NamedTensorRef + MutableBufferSpec, merged): binds a
  placeholder ValueRef to external storage. key is the namespace-3 fqn (an
  external-constant-file key when has_data, else the buffer's cross-method
  identity); has_data selects the load path (fetch bytes vs zero-init at
  load); mutated marks a Buffer written in place whose state persists across
  executions; role mirrors the bound value's role for direct access when
  iterating bindings.

• OutputKind (graph-level: UserOutput / BufferMutation / UserInputMutation) and
  OutputSpec (aligned to graph.output_refs by index). A mutation output's
  target_ref is a ValueRef to the mutated placeholder value in this graph
  (both the mutated user input and the mutated buffer are lifted placeholders
  here), replacing the schema's dual-namespace target string; the wire fqn /
  SSA name is recovered from that value.

• Method itself: name, Graph graph, data_bindings, output_specs, plus a
  to_string() debug dump. HOP subgraphs carry no bindings (their params are
  lifted here and passed as operands), which is why Method wraps Graph
  rather than folding into it.

Pure std only (no ExecuTorch, no flatbuffers).

Differential Revision: D114426388

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22104

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❌ 1 New Failure

As of commit 5441f30 with merge base 9a2d135 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  adb: failed to install android-test-debug-androidTest.apk: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE: Existing package org.pytorch.executorch.test signatures do not match newer version; ignoring!]

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