Support multimethod in runner

[lucylq]

Stack from ghstack (oldest at bottom):

• Introduce LoraConfig #17229
• -> Support multimethod in runner #17228

Specify which method to call in the runner.

Update CLAUDE.md for fbcode/xplat.

Differential Revision: D92225533

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17228

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❌ 1 New Failure, 1 Unrelated Failure

As of commit a0cea55 with merge base aa2f683 ():

NEW FAILURE - The following job has failed:

• Test CUDA Builds / export-model-cuda-artifact (openai, whisper-large-v3-turbo, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t 26441df61ba787f798d81aa6834eb5c5be29777654fd78e224ab75ee7eae5379 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.arm64.2xlarge, executorch-ubuntu-22.04-... / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: Head "https://registry-1.docker.io/v2/arm64v8/alpine/manifests/latest": received unexpected HTTP status: 500 Internal Server Error.

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[Copilot]

Pull request overview

This PR adds multimethod support to the LLM runner infrastructure, allowing users to specify which method to execute in the model instead of hardcoding "forward". This is part of a larger stack implementing multimethod and LoRA support.

Changes:

• Added method_name parameter to TextDecoderRunner and related factory functions, with "forward" as the default
• Updated all runner code to use the configurable method name instead of hardcoded "forward"
• Added command-line flag --method_name to the llama example
• Included minor documentation updates to CLAUDE.md about fbcode/xplat conventions

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
extension/llm/runner/text_decoder_runner.h	Added method_name constructor parameter, member variable, and getter method
extension/llm/runner/text_decoder_runner.cpp	Updated constructor and step() to use configurable method_name; fixed typo in error message
extension/llm/runner/test/test_text_decoder_runner.cpp	Added tests for default and custom method names
extension/llm/runner/llm_runner_helper.h	Added method_name parameter to create_text_llm_runner function signatures
extension/llm/runner/llm_runner_helper.cpp	Propagated method_name through factory functions; added logging for method name
examples/models/llama/runner/runner.h	Added method_name parameter to create_llama_runner functions
examples/models/llama/runner/runner.cpp	Propagated method_name through create_llama_runner implementations
examples/models/llama/main.cpp	Added --method_name command-line flag
CLAUDE.md	Added documentation about fbcode/xplat directory structure; minor formatting fixes

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