Enable MI300X ROCm support

[ehartford]

This PR enables DeepSeek V4 Pro to run on AMD Instinct MI300X by adding CDNA-oriented ROCm kernels and sharding model layers across local ROCm GPUs.

Summary

• Add CDNA3/CDNA4 direct MFMA wrapper kernels for f16 MFMA:
  - gfx942 uses mfma_f32_16x16x16_f16
  - gfx950 uses mfma_f32_16x16x32_f16

• Add a CDNA Q8 batch matmul/MFMA prefill path.

• Add ROCm MoE kernel fixes for CDNA correctness, including disabling the broken IQ2/Q2 float-down WMMA overlay.

• Add ROCm attention/activation fixes to avoid fp16 overflow and repeated BOS failures.

• Add MI300X/CDNA build targets, with CDNA4 gfx950 compile plumbing.

• Add local --gpus launcher using the existing distributed runtime to shard layers across local GPUs.

• Support repeated -m model shards independent of argument order.

• Allocate graph/KV/cache state only for the layer slice owned by each worker.

• Add model-cache preflight checks for early actionable OOM errors.

• Use BF16 for 16-bit distributed activation transport.

• Add MI300X/ROCm smoke scripts and a synthetic Q8 MFMA correctness test.

Validation

Validated on MI300X / CDNA3:

make mi300x
git diff --check

Also validated a local sharded Pro Q4 run across MI300X GPUs, including reversed -m shard order.

Notes

CDNA4 / gfx950 kernel selection and build plumbing are included, but runtime validation has not been performed because I do not have CDNA4 hardware.

[beverm2391]

+1 waiting on this one!