Add grouped attention for variable-length sequences in SA3 wrapper

[jtreminio]

Attempting to use SageAttention3 for an image upscale fails every time due to uniform sequence length requirements, unless I have SA2 as a fallback. However, this was both a silent fallback (this PR adds logging), and I figured we could actually fix the implementation to allow SA3 to work.

• grouping sequences by length
• processing each group with SA3
• reassembling outputs

Changes:

• uniform path: Unchanged, when all sequences have the same length, uses direct batching
• variable-length path: groups sequences by length and processes each group with SA3
• logging: logs when grouped attention is used and when fallback occurs