add small zimage test and fix bug

[xin3he]

Description

This pull request introduces support for testing and handling a new diffusion model, "Z-Image", and makes improvements to model file renaming and test fixture setup. The main changes include adding a test fixture and test case for the Z-Image model, updating the weights file renaming logic, and refining how model modules are loaded in tests.

Support for Z-Image diffusion model:

• Added a new pytest fixture tiny_z_image_model_path in test/fixtures.py to create and clean up a minimal Z-Image model for testing, with custom configuration overrides.
• Introduced a new test test_z_image_tune in test/test_cuda/models/test_diffusion.py to verify quantization and saving for the Z-Image model using the new fixture.

Model file handling improvements:

• Updated rename_weights_files in auto_round/utils/model.py to handle the case where there is only one .safetensors file by renaming it directly, avoiding unnecessary enumeration.

Test utilities and module loading:

• Modified _get_module in test/helpers.py to directly set reduced modules for specific model attributes, improving how models with nested modules are handled in test environments.

Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Code refactoring
• Other (please specify):

Related Issues

Fixes or relates to #

Checklist Before Submitting

• My code has been tested locally.
• Documentation has been updated as needed.
• New or updated tests are included where applicable.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a CUDA test + fixture for a minimal “Z-Image” diffusion model and adjusts tiny-model module construction and safetensors renaming behavior.

Changes:

• Add tiny_z_image_model_path fixture and a test_z_image_tune CUDA test.
• Update tiny-model helper to replace certain nested modules with reduced “tiny” modules.
• Adjust rename_weights_files to special-case single .safetensors files.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/test_cuda/models/test_diffusion.py	Adds a CUDA test exercising Z-Image quantization/saving path.
test/helpers.py	Changes tiny-model construction to replace nested modules rather than mutating configs in-place.
test/fixtures.py	Adds a session-scoped fixture that builds a minimal Z-Image model with config overrides.
auto_round/utils/model.py	Special-cases renaming when only one .safetensors file exists.

[xin3he]

/azp run Unit-Test-CUDA-AutoRound

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