[Bugfix][Quant] Raise actionable error instead of bare assert for group-size/TP mismatch (#46230)

[ArsalanShakil]

Purpose

Fixes #46230.

Loading a group-quantized compressed-tensors checkpoint (W4A16 / W8A16, and the W4A8-FP8 scheme) could abort at model-load time with a bare, message-less:

AssertionError: assert input_size_per_partition % group_size == 0

This fires when tensor-parallel sharding splits a layer's input dimension into a shard that is not a whole number of quant groups (input_size_per_partition = input_size // tensor_parallel_size not divisible by group_size). The user is left with a cryptic assertion and no idea what to change.

Fix

Extract the divisibility check into a single helper, verify_group_size_divides_partition, in marlin_utils.py (mirroring the existing verify_marlin_supports_shape), and have it raise a descriptive ValueError that names the layer, the offending sizes, and the remedy. The three compressed-tensors schemes that previously had identical bare asserts now call it:

• compressed_tensors_wNa16.py
• compressed_tensors_wNa8o8.py
• compressed_tensors_w4a8_fp8.py

Resulting error:

Cannot load group-quantized weights for layer 'model.layers.0.mlp.down_proj':
input_size_per_partition=320 is not divisible by group_size=128. This happens
when tensor_parallel_size splits the layer input into shards that are not a
whole number of quant groups. Consider reducing tensor_parallel_size.

This aligns these schemes with the actionable error style already used in verify_marlin_supports_shape ("Consider reducing tensor_parallel_size ...").

Not a duplicate

Checked the issue timeline and open PRs: there is no existing PR addressing #46230.

Test

Added tests/quantization/test_group_partition_divisibility.py covering both the non-divisible (raises actionable ValueError) and divisible (no raise) cases.

$ python -m pytest tests/quantization/test_group_partition_divisibility.py -q
2 passed

Lint:

$ ruff check <changed files>      # All checks passed!
$ ruff format --check <changed files>   # clean

Note on verification scope: the unit test exercises the extracted helper directly. The full create_weights path could not be run locally because kernel selection (choose_mp_linear_kernel) requires a GPU platform; the scheme wiring is a 1:1 replacement of the previous assert and is left for CI / GPU review.

This change was developed with AI assistant help; I have reviewed every line and run the tests above.

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[whyseu]

Any progress?

[ArsalanShakil]

@whyseu Thanks for the ping! The fix is ready on my end, it's just waiting on a maintainer to add the ready label so CI can run and review can start. I'll follow up as soon as there's an update.

[vadiklyutiy]

It looks very similar to check_marlin_supports_shape().
Is there duplication here?

[ArsalanShakil]

@vadiklyutiy There was overlap on the group-size divisibility check. I pulled that into a shared verify_group_size_divides_partition helper and had verify_marlin_supports_shape delegate to it, so there's now a single source of truth (the Marlin thread_n/thread_k checks and the group_size < input_size guard stay in verify_marlin_supports_shape).

I didn't reuse verify_marlin_supports_shape directly from the compressed-tensors schemes, since that branch runs for any kernel (not just Marlin) and pulling in the Marlin thread constraints would reject valid non-Marlin shapes.

[hmellor]

I like the idea, but IMO this is more of a tp/distributed utility than a marlin utility.

Could you make sure that no such distributed utility doesn't already exist, and then use the distributed utility instead?

You may also be able to find other places where bare asserts can be replaced with this utility

[hmellor]

Thanks for improving the DX!

[hmellor]

(please fix DCO and we'll be able to merge)

[ArsalanShakil]

@hmellor DCO is now solved

[hmellor]

Please stop merging from main, if there are flaky tests I will rerun them individually

[ArsalanShakil]

The one red job (amd-entrypoints-integration-api-server-openai) failed only on test_shutdown.py::test_request_rejection_during_shutdown — 104 passed, 1 failed. The request-rejection assertion itself passed; it tripped on the post-SIGTERM cleanup check:

Failed: Child processes [30125] still alive after 10s. Process cleanup may not be working correctly.

One engine child process didn't exit within the 10s timeout on ROCm. This is unrelated to the change here — the diff only adds the verify_group_size_divides_partition helper in distributed/utils.py and switches three compressed-tensors schemes from a bare assert to it; nothing touches the API server, process lifecycle, or signal handling. Looks like a timing flake on the AMD shutdown test.