Fix CUDA low-memory external weights export

[Gasoonjia]

Summary

• Stream AOTInductor external weights to disk instead of materializing a model-sized bytes object in host memory.
• Keep the resulting weights blob file-backed during PTE serialization.
• Avoid deep-copying immutable tensor storage for each delegated method.
• Move compile-time clones of mutated buffers to CPU and temporarily release zero-initialized KV-cache storage.
• Preserve the existing export path unless low_memory_mode is enabled.

On MG 30B, peak host RSS decreased from 75.2 GiB to 48.8 GiB, a 26.3 GiB (35.1%) reduction. A separate 131K-context export completed with approximately 17 GiB peak GPU.

Testing

• Added CUDA low-memory export and serialization unit tests.
• Verified full MG 30B export and runtime successfully.

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure, 4 Unclassified Failures

As of commit 72243ad with merge base e60faa2 ():

NEW FAILURES - The following jobs have failed:

• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, Qwen3.5-35B-A3B-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• Test CUDA Builds / test-executorch-cuda-build-12.6 / linux-job (gh)
  RuntimeError: Command docker exec -t 7f236d3934dc1c63ccf7c4a7e4527dc88e8c313bba14a3e117cf8b6e6285749e /exec failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Test CUDA Builds / test-muse-glimmer-cuda-e2e-kquant-17gb-dflash-image / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• Test CUDA Builds / test-muse-glimmer-cuda-e2e-kquant-17gb-solo-text / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• Test CUDA Builds / test-muse-glimmer-cuda-e2e-kquant-dynamic-dflash-image / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1
• Test CUDA Builds / test-muse-glimmer-cuda-e2e-kquant-dynamic-solo-text / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1

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

• Test CUDA Builds / check-all-cuda-builds (gh) (detected as infra flaky with no log or failing log classifier)

This comment was automatically generated by Dr. CI and updates every 15 minutes.

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