[Bugfix][ROCm] Use batch DMA for CPU KV cache loads

[AndreasKaratzas]

Summary

• Add ROCm to the existing XPU batch-DMA fallback for CPU-to-GPU KV transfers.
• Keep CUDA, XPU, and every other prior dispatch path unchanged.

The ROCm Triton path directly loaded a raw shared-mmap host pointer. The reproduced fault address matched the mmap base plus the selected block offset in the first case above the Triton descriptor threshold.

Validation

• Complete KV offload file: 25 passed on MI355.
• The former failure passed the full 16-case matrix on two GPUs and 100 same-process repeats.
• Changed-file pre-commit passed.

Buildkite: V1 Core + KV + Metrics

Duplicate check: no open PR addresses ROCm host-mapped KV loads; #41790 only adds separate offload metrics.

AI assistance: Codex assisted with investigation, implementation, and validation; Andreas directed the minimal platform dispatch and reviewed the resulting changes.