[wip] Introduce MultimethodLoraConfig

[lucylq]

Stack from ghstack (oldest at bottom):

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

Differential Revision: D92315627

[pytorch-bot]

🔗 Helpful Links

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

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❌ 3 New Failures, 2 Unrelated Failures

As of commit c918989 with merge base eac0673 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for extension/llm/export/config/llm_config.py:
• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a42f12a832747ff35dc73609b808c1317a84daf907634311c67d10e329710dbf /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 57696123c445292cf565da1d7c194448fdd27ca6e79c588d196e67f434c2be8f /exec failed with exit code 1

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

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

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