[C] Separating cudnn common utils from fused_attn

[phu0ngng]

Description

In this PR, the common cudnn helpers are moved out of fused attention so that they can be used in other kernels, i.e. the coming normalization.

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactor

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[phu0ngng]

/te-ci pytorch

[cyanguwa]

LGTM

[phu0ngng]

/te-ci pytorch