Add CoreML backend options with compute unit configuration (#18369)

[metascroy]

Summary:

This diff adds type-safe backend options support to the CoreML delegate, allowing users to configure compute units (CPU, GPU, Neural Engine) at model load time using the new LoadBackendOptionsMap infrastructure.

Key changes:

• Added LoadOptionsBuilder class in coreml_backend_options.h providing a fluent API for setting CoreML options with compile-time type safety
• ComputeUnit enum nested inside the builder for type-safe compute unit selection (CPU_ONLY, CPU_AND_GPU, CPU_AND_NE, ALL)
• Integrated runtime spec retrieval in backend_delegate.mm to read compute_unit option from BackendInitContext
• Added comprehensive unit tests for the new options builder

Example usage:

using executorch::backends::coreml::LoadOptionsBuilder;

LoadOptionsBuilder coreml_opts;
coreml_opts.setComputeUnit(LoadOptionsBuilder::ComputeUnit::CPU_AND_GPU);

LoadBackendOptionsMap map;
map.set_options(coreml_opts);

module.load(method_name, map);

Differential Revision: D92358632

[pytorch-bot]

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❌ 1 New Failure, 42 Pending

As of commit 73c5fdb with merge base fd1581d ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t d98909dc856aa520bd5c2faeae4c4e2504982149af0857132fbadcc574cf350a /exec failed with exit code 1

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[meta-codesync]

@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D92358632.

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