Save external constant tensors to custom filename

[lucylq]

Summary: Adding a Callable option in the config, so we can customize the nodes to save, and the filenames to save to.
The pass is applied in _program.py, it's hard to do this on the eager model (like we do with delegates) as we have to run the pass after SpecPropPass.

Differential Revision: D87280747

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job, 4 Unrelated Failures

As of commit 96efb61 with merge base d2c011e ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• pull / unittest / linux / linux-job (gh)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module
• pull / unittest-editable / linux / linux-job (gh)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

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

• pull / unittest-wasm-bindings / linux-job (gh)

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

• periodic / test-models-linux (cmake, vit, portable, linux.2xlarge, 90) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-static-llama-qnn-linux (stories_260k_bc) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / unittest-wasm-bindings (--enable-etdump) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• Test CUDA Builds / test-models-cuda (resnet18) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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[meta-codesync]

@lucylq has exported this pull request. If you are a Meta employee, you can view the originating Diff in D87280747.

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[Copilot]

Pull Request Overview

This PR adds support for customizing external constant tensor storage by allowing a Callable option in the ExecutorchBackendConfig.external_constants field. Previously, this field only accepted a boolean value to determine whether all constants should be externalized. Now it can also accept a function that determines which constants to externalize and which filename to use for storage.

Key changes:

• Extended external_constants to accept either bool or Callable[[torch.fx.Node], Optional[str]]
• Updated the conditional logic in to_executorch() to handle both bool and callable cases
• Added test coverage for the custom callable scenario

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
exir/capture/_config.py	Extended type annotation and documentation for external_constants to support Callable option
exir/program/_program.py	Updated conditional logic to handle both bool and callable external_constants values
exir/emit/test/test_emit.py	Added test case for custom callable external_constants behavior

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