Introduce MultimethodLoraConfig

[lucylq]

Stack from ghstack (oldest at bottom):

• Support multimethod in export_llama_lib #17231
• -> Introduce MultimethodLoraConfig #17230
• Introduce LoraConfig #17229
• Support multimethod in runner #17228

Differential Revision: D92315627

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17230

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❌ 16 New Failures, 1 Cancelled Job

As of commit 9c941c7 with merge base efe4f0c ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (ios-simulator) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build Presets / apple (ios) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build Presets / apple (llm) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build Presets / apple (macos) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 0a94bf83caf784a8f918f86347f7881b184cfc17bf55039c82f8416972c72663 /exec failed with exit code 1
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f05ca4057ca9933d5814a2a3c94a26460d8a502a4599a715197e89cdde045df1 /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1031260955f6ac88e13f64831a9adf6ccdea236a675576ed42603baca33f477e /exec failed with exit code 1
• Test CUDA Builds / test-cuda-pybind (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t c07d408b8902e8220fa07dbe8ea429f11d5caf5b3deab9862c94f6b9435fcf91 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 4a657b2a70f379a70ab6e6190d315e18bee594af7745c58a507e656b4af93710 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 96aca94fff1f07e96c7f1e6f2c18910666ecb37590ba0fc3ce55891ff76ae099 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t ac202f21fbf5c10c17a233b22002af4b108cb596cf8e7175d741b6177f2ffd43 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t a95a8e6caae7510d63f8b16ccd49e5fb7a53cf98c0b8ac929b4812fa81d44311 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t ccfc154e1823d97775c0830d3fa983f031cb3c86930898f5bb37a60c131278a2 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t bbc8a32b8b643619adfb78c0152847c336f73dc6938412ccb3552c2e2dc0c902 /exec failed with exit code 1
• Test Metal Backend / test-model-metal-e2e (openai, whisper-large-v3-turbo, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / test-model-metal-e2e (openai, whisper-small, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / test-samsung-quantmodels-linux / linux-job (gh)
  ##[error]The operation was canceled.

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[Copilot]

Pull request overview

This PR introduces MultimethodLoraConfig, a new configuration class that enables exporting multiple methods (e.g., base model and LoRA variants) to a single .pte file. This is part of a larger stack adding multimethod support to the LLM export pipeline.

Changes:

• Added Dict to typing imports to support the new configuration structure
• Introduced MultimethodLoraConfig class with a dictionary mapping method names to optional LoRA configurations
• Integrated MultimethodLoraConfig as a new field in the LlmConfig class

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