Add regression tests for hooks, schedulers, and shared utilities

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What does this PR do?

Adds fast, deterministic regression tests for high-blast-radius code paths that previously had weak or no direct unit coverage.

Also fixes utils/check_copies.py so copy consistency checks stylify both observed and theoretical blocks before comparing (avoids false drift when only one side was ruff-formatted).

Risky behavior now covered

• FirstBlockCache skip logic — threshold-based tail-block skipping, tuple (Flux-style) outputs, and recomputation after input change
• AmusedScheduler — Gumbel masking helpers, 2D tensor reshaping, intermediate/final timestep behavior, invalid schedule validation
• CogVideoX schedulers — SNR shift, set_timesteps guards, multi-step DPM API
• DDPMWuerstchenScheduler — float timestep schedule and add_noise
• FlowMatchEulerDiscrete / Helios / LTXEulerAncestralRF — non-standard step APIs and multi-stage scheduling
• loading_utils / remote_utils / state_dict_utils — path/URL validation, submodule resolution, remote decode guards
• DDIM / DDPM — additional edge-case assertions from prior coverage pass

Test files added/updated

• tests/hooks/test_first_block_cache.py (new)
• tests/schedulers/test_scheduler_amused.py (new)
• tests/others/test_loading_utils.py (new)
• tests/others/test_remote_utils.py (new)
• tests/others/test_state_dict_utils.py (new)
• tests/schedulers/test_scheduler_cogvideox.py (new)
• tests/schedulers/test_scheduler_ddpm_wuerstchen.py (new)
• tests/schedulers/test_scheduler_flow_match_euler_discrete.py (new)
• tests/schedulers/test_scheduler_helios.py (new)
• tests/schedulers/test_scheduler_ltx_euler_ancestral_rf.py (new)
• tests/schedulers/test_scheduler_ddim.py, test_scheduler_ddpm.py (extended)
• utils/check_copies.py (comparison fix)

Why these tests materially reduce regression risk

• FirstBlockCache is shared inference acceleration logic used across multiple pipelines; incorrect skip/recompute behavior silently changes outputs.
• AmusedScheduler has a bespoke discrete-token API (not covered by SchedulerCommonTest); masking schedule bugs would break Amused pipelines entirely.
• Utility loaders (load_image, get_submodule_by_name, remote decode) sit on critical paths for pipeline I/O; bad path handling or state-dict parsing causes hard-to-debug load failures.
• Specialized schedulers (CogVideoX, Wuerstchen, Helios, LTX RF) have custom step signatures; pipeline tests alone do not lock in scheduler contract details.

Validation

PYTHONPATH=src python3 -m pytest \
  tests/hooks/test_first_block_cache.py \
  tests/schedulers/test_scheduler_amused.py \
  tests/others/test_loading_utils.py \
  tests/others/test_remote_utils.py \
  tests/others/test_state_dict_utils.py \
  tests/schedulers/test_scheduler_cogvideox.py \
  tests/schedulers/test_scheduler_ddpm_wuerstchen.py \
  tests/schedulers/test_scheduler_flow_match_euler_discrete.py \
  tests/schedulers/test_scheduler_helios.py \
  tests/schedulers/test_scheduler_ltx_euler_ancestral_rf.py \
  -q

90 tests passed locally.

• Did you use an AI agent to help with this PR?
• Did you write any new necessary tests?