Allow mixed ConvRot groups in MiniMax H3 checkpoints

[bghira]

Summary

• preserve the per-layer ConvRot Hadamard group size when loading MiniMax H3 checkpoints
• describe mixed group sizes in quantization metadata instead of rejecting the checkpoint
• cover mixed 64/256 group checkpoints in the single-file loader test

Validation

• loaded the released 34 GB MiniMax H3 FL2VA INT8 ConvRot checkpoint containing group sizes 64 and 256
• validated FastVideo Dense and both Tutu acceleration adapters against the loaded model on H100
• pre-commit hooks passed