Load a method whose constant name collides with another method's

[shoumikhin]

The problem

A program can hold several methods, for example one that fills a cache from a prompt and one
that runs a single step. The CUDA delegate can share a constant between them so one copy of a
weight serves every method that uses it. Sharing is keyed on the constant's name.

That key is not unique. A constant lifted out of a graph is named by its position within one
compiled module, so the first method's third constant and the second method's third constant are
both called _tensor_constant2, and they usually hold different data. Loading such a program
fails:

Constant '_tensor_constant2': size mismatch at dim 0 (cached=8448, new=6656)
Constant '_tensor_constant2' in method 'decode' is incompatible with the cached version
  from a previous method. Refusing to share.

The program is fine. Each method carries its own copy of the constant and can run with it. Only
the attempt to treat two unrelated tensors as one fails.

The change

Refusing to alias mismatched storage is correct, so that check stays. What changes is the
response: decline to share that one constant and let the method use its own copy, rather than
failing the load for the whole program. Constants that do match go on being shared exactly as
before, so a program with no collisions behaves identically.

One detail matters for correctness. The cache lookup runs before the tensors can be compared, so
a name match is only provisional at that point. A constant rejected after extraction has to be
left out of the user-managed pairs too, otherwise the method is pointed at a tensor it did not
ask for.

Scope

This fixes the load failure and nothing else. On the program used to test it, the run is not yet
deterministic across repeats, and that is also true before this change when sharing is turned
off, so it is a separate pre-existing issue rather than something this introduces or resolves.

Testing

Built the CUDA delegate from source on Linux x86_64 and ran a three-method program whose methods
have colliding constant names, comparing the same program and runner against an unmodified
delegate:

unmodified, sharing on:   load fails, no method runs
unmodified, sharing off:  loads, all three methods run
this change, sharing on:  loads, all three methods run

The first row is the failure above. The second shows the program itself is loadable, which is
what makes failing the load the wrong response.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21706

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❌ 3 New Failures, 3 Unrelated Failures

As of commit daf2840 with merge base 48741ac ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/cuda/runtime/cuda_backend.cpp:
• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9ffa9a215d83485330bfb4644b792b951b5be1fa6bd463c35d00a51cecce0fe9 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 020b7ffb2eff73db55e57588bd23aff068195b9675642806ef6383e4827c49a3 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  extension/llm/modules/test/test_turboquant_kv_cache.py::TestCodebook::test_rotation_varies_with_seed
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  extension/llm/modules/test/test_turboquant_kv_cache.py::TestCodebook::test_rotation_varies_with_seed
• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  test_concat_cluster_quantization__conv_7_sigmoid

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