NXP Backend: Add option for prefetching weights from external memory

[jirioc]

Summary

Models that do not fit into SRAM can be placed into external memory. Since the Neutron has no access to external FLASH, the firmware needs to prefetch the weights from FLASH to SRAM in advance. This commit instructs the Neutron Converter to create extra instructions for such prefetching. Each prefetching is started in parallel with the compute, but can take longer, so this feature needs to be used only when necessary.

This commit changes only the AoT part, the necessary runtime changes are yet to be implemented in different PR.

Test plan

Unit test added to backends/nxp/tests/test_neutron_converter_manager.py

cc @robert-kalmar @JakeStevens @digantdesai

[pytorch-bot]

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❌ 1 Awaiting Approval, 4 New Failures, 1 Unrelated Failure

As of commit e7feb15 with merge base 059c729 ():

AWAITING APPROVAL - The following workflow needs approval before CI can run:

• periodic (gh)

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t acdc3a278dbab2db79befc8a2a1a447113f8ae937ac51d6ab7d6e38a94db7cc0 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t db2c9aea150dfe47a2a2f094fdf149539650e78976b44f7b1a1de97ff4c1b8b6 /exec failed with exit code 1
• pull / test-sqnr-static-llm-qnn-linux (smollm2_135m) / linux-job (gh)
  RuntimeError: Command docker exec -t 307fa448f2aa47c9d561e060fff51c03877d95cdff23574e57353ccbdf5f8c53 /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t d3d2a234bb9900d63650eb727bc9637f1051aac0250dedc8681c6dd6c99f912d /exec failed with exit code 1

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

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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