[tx] Implement cutlass kernel for ragged_dot with group_offset

[pcmoritz]

This brings down the step time of

uv run --with wandb --with tinker==0.3.0 sl_loop.py     base_url=http://localhost:8000     model_name=Qwen/Qwen3-30B-A3B lora_rank=1 max_length=512

with

uv run --extra gpu --extra tinker -m tx.tinker.api     --base-model Qwen/Qwen3-30B-A3B     --backend-config '{"max_lora_adapters": 2, "max_lora_rank": 8, "expert_parallel_size": 8, "train_micro_batch_size": 1, "shard_attention_heads": false}'

from 40s to 20s. I spend some time tuning the tile sizes and also tried different tile sizes / configurations for different settings (e.g. the different projections or low k setting for LoRA), but it only made a very small difference and wouldn't be worth the complexity for now.