Pass share_mutable_buffers through llm_config

[lucylq]

Summary: Support in export_llama_lib and internally.

Differential Revision: D94456166

[pytorch-bot]

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❌ 3 New Failures, 4 Unrelated Failures

As of commit 176c8df with merge base 5f879ca ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f2a29d28abb1bb4fe1ac29df8019f9080cb32b4dd2a238468b008cdae8a27a07 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t aa460b2b3ae78e7691f089df472964fe698a7437911bb552b87d934fd64a06f7 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux-basic (vit, portable, cmake, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, cmake, linux.2xlarge, executorch-u... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-setup-linux-gcc / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-nxp-neutron / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[Copilot]

Pull request overview

This PR adds support for configuring share_mutable_buffers via llm_config for multimethod LoRA exports, and plumbs the flag into the ExecuTorch memory planning stage so mutable buffers can be shared across entry points.

Changes:

• Extend MultimethodLoraConfig with a share_mutable_buffers flag.
• Add a share_mutable_buffers parameter to LLMEdgeManager.to_executorch() and pass it into MemoryPlanningPass.
• Wire the config flag through export_llama_lib.py multimethod export and update the Qwen3 multimethod example YAML.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
extension/llm/export/config/llm_config.py	Adds share_mutable_buffers to the multimethod LoRA export config schema.
extension/llm/export/builder.py	Plumbs share_mutable_buffers into MemoryPlanningPass during lowering to ExecuTorch.
examples/models/qwen3/config/qwen3_multimethod.yaml	Demonstrates enabling mutable buffer sharing in a multimethod config.
examples/models/llama/export_llama_lib.py	Passes llm_config.multimethod_lora.share_mutable_buffers into the export pipeline for multimethod builds.

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