Fixed layernorm when weight and bias is None in Stable Diffusion 3

[cehongwang]

Description

Fixed layernorm when weight and bias is None in Stable Diffusion 3

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ x] My code follows the style guidelines of this project (You can use the linters)
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas and hacks
• [ x] I have made corresponding changes to the documentation
• [ x] I have added tests to verify my fix or my feature
• [ x] New and existing unit tests pass locally with my changes
• [ x] I have added the relevant labels to my PR in so that relevant reviewers are notified

[narendasan]

Can you add a test case if possible?

[narendasan]

LGTM