Add Group Offloading Summary Utility

[dg845]

What does this PR do?

This PR adds a group offloading summary utility _get_group_offload_summary which outputs a string showing the offloading groups of a module. The motivation is to allow users to easily see the created offloading groups and to aid in debugging group offloading bugs. It can also be used for more informative test messages (an example is in GroupOffloadTesterMixin._run_group_offload_inference in this PR).

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[dg845]

Self-Review Report

Self-review

Ran the /self-review rubric (.ai/references/review-rules.md, plus code_style.md and testing.md) over the full branch diff. Verdict: ready. No blocking issues remain; the three items below are judgement calls I'd rather put to a reviewer than settle alone.

What this adds

commit	what
Add a group offloading introspection helper	_get_group_offload_summary in src/diffusers/hooks/group_offloading.py, plus its use in the pipeline group-offload tests
Test the group offloading summary	four tests in tests/hooks/test_group_offloading.py

Group offloading installs its grouping on the GroupOffloadingHook in each submodule's _diffusers_hook registry, several submodules share one ModuleGroup, and groups hold module objects rather than names. So when a group's weights are never onloaded, the resulting error names only the layer that tripped over it. The helper renders the grouping; _run_group_offload_inference attaches it to any RuntimeError from the offloaded run.

Each group is reported against the module whose forward actually brings its weights over. That is not always its own leader — with use_stream=True, lazy prefetch sets onload_self = False and the group is onloaded by whichever group names it as next_group:

use_stream=False                          use_stream=True, after the first forward
  onloaded by '<root>'   [head]             onloaded by '<root>'      [head]
  onloaded by 'blocks.0' [blocks.0]         prefetched by '<root>'    [blocks.0]
  onloaded by 'blocks.1' [blocks.1]         prefetched by 'blocks.0'  [blocks.1]
  onloaded by 'blocks.2' [blocks.2]         prefetched by 'blocks.1'  [blocks.2]

Non-blocking findings

1. The test oracle shares the implementation's algorithm. installed_groups (tests/hooks/test_group_offloading.py:721) reimplements the same registry walk and id(hook.group) dedupe as the code under test, so a shared misconception about which hooks exist would be invisible to test_reports_one_line_per_installed_group. It is genuinely independent where it matters most: test_reports_prefetching_only_once_the_chain_is_wired reads group.onload_self straight off the hook, while the implementation derives prefetching through a separate map. Inherent to the domain rather than fixable.

2. TestGroupOffloadSummary has no teardown_method. It is the only accelerator-touching class in the file without the gc.collect() / backend_empty_cache / backend_reset_peak_memory_stats teardown that TestGroupOffload has. I checked whether this can contaminate the file's memory assertions and it cannot — test_offloading_forward_pass's inner run_forward resets peak stats before each measurement and compares relatively, so a constant residual preserves the inequalities. Consistency, not a flake.

3. The handler decorates every RuntimeError, not only device mismatches. tests/pipelines/testing_utils/memory.py:377. A shape error or a cuDNN gap also gets an offload-grouping block appended. Harmless — the grouping is factual regardless, and the original message is preserved as a prefix so skip_if_no_cudnn_engine's substring match still works — but noisier than the comment implies.

Fixed during review

• The summary misreported prefetched groups. It printed onloaded by <own leader> unconditionally. Measured on a 3-block model under use_stream=True, three of four lines were wrong, on exactly the path where a device mismatch is hardest to reason about.
• KeyError when summarizing a submodule. name_of is built from the walked module, but a group with num_blocks_per_group > 1 holds that module's siblings, so the lookup missed. Calling the helper on one component of an offloaded model — the natural thing to do while debugging — crashed. Members outside the walked module are now reported as <outside this module>.
• Five of nine fields on the original GroupOffloadGroupInfo dataclass were never read. Inlining the walk into the single consumer removed the dataclass and the dead fields, and turned onload_self from unused into load-bearing.
• A second return value the only caller discarded, in _enable_group_offload_on_components.

What the tests actually pin

The helper only runs once something has already failed, so a regression in it is invisible — a summary is still produced, it is just wrong, and it sends the reader to the wrong module. Nothing else in the suite would notice. Each fix was reverted in turn to confirm the tests catch it:

reverted fix	test that fails
direct name_of[...] lookups (the KeyError)	test_summarizes_a_submodule_whose_group_reaches_outside_it
unconditional "onloaded by"	test_reports_prefetching_only_once_the_chain_is_wired
silently dropping unnameable members	test_summarizes_a_submodule_whose_group_reaches_outside_it

Each fails exactly one test and leaves the others green. The tests assert against groups read straight off the hooks; only "no group offloading applied" and "prefetched by" are matched as text, so the layout stays free to change.

Open questions

Should the hooks / groups split collapse? The walk keeps every hook to build the prefetch chain and dedupes only for display. That distinction is provably unobservable today: it needs a group with more than one hook and a prefetch chain, and group_offloading.py:723 coerces num_blocks_per_group = 1 whenever a stream is used, so the two never coexist. Collapsing the lists fails no test. I kept the general form because it is written against the data structure — next_group is a hook attribute, group is shared — rather than against a coercion 600 lines away that exists for unrelated reasons, and because if that restriction is ever lifted the deduped version silently reports prefetched groups as self-onloading. code_style.md's "no unused code paths" reads the other way, so I would take direction here.

Should the leading underscore come off? The original argument for keeping it private was that GroupOffloadGroupInfo mirrored ModuleGroup's fields and would freeze an implementation detail. That dataclass is gone; the function returns a plain str, whose content can change freely. If reaching for this while debugging a component is an intended use, the underscore is a sign saying "don't" on the thing people would want. Making it public would mean a hooks/__init__.py export and an entry in docs/source/en/api/utilities.md alongside apply_group_offloading.

Should there be a test for the exclusion lists? An earlier draft added one asserting that a config's block-level exclusions are entirely root-gated. I dropped it: the predicate is unsound. PriorTransformer splits its transformer_blocks into per-block groups yet reads clip_mean / clip_std from the root group in post_process_latents (18 of 50 parameters root-gated), and AutoencoderKL groups entirely into its down_blocks / up_blocks while decode bypasses forward (0 of 92). Both have exactly the bug the exclusions exist for, and both would have failed that check — it would have told you to remove a load-bearing exclusion. Whether a component is safe to offload at block level depends on which entry points the pipeline drives it through, which the module graph alone does not say.

Verification

• tests/hooks/test_group_offloading.py — 42 passed
• tests/pipelines/{ideogram4,pndm,ltx2} with -m "memory or group_offload" — 82 passed, 2 skipped, 11 xfailed, 1 xpassed (the xpass is the pre-existing documented pndm one)
• ruff format and ruff check clean

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