[common] Misc improvements for attention

[cyanguwa]

Description

This PR:

• removes nvte_fused_attn_fwd_qkvpacked, nvte_fused_attn_bwd_qkvpacked, nvte_fused_attn_fwd_kvpacked, nvte_fused_attn_bwd_kvpacked APIs, and leaves nvte_fused_attn_fwd and nvte_fused_attn_bwd for uniformity and easier maintainance,
• improves error messaging in nvte_get_fused_attn_backend (ultimately the backend selection logic in this function will be replaced by cudnn-frontend's check_support() when it's ready).

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 refactoring

Changes

See description.

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

[cyanguwa]

/te-ci L0

[cyanguwa]

/te-ci L0

[cyanguwa]

/te-ci L0

[pggPL]

This PR removes API which is exposed in the docs in advanced/c++ API. In my PR I tried not to remove it - as I discussed with @ptrendx. So if you think that it can be removed, I will close my PR. If not, then maybe we should merge my PR in first place, and then merge it with yours.