ggml : fix tensor-parallel + -ncmoe crash on MoE models (#25028)

Tensor parallelism (-sm tensor) combined with -ncmoe (CPU-offloaded MoE experts) aborts during warm-up on MoE models with GGML_ASSERT(ggml_is_contiguous(tensor)) in ggml-backend-meta.cpp.

The failing tensor is the MoE router output (ffn_moe_topk): it is mirrored (GGML_BACKEND_SPLIT_AXIS_MIRRORED, replicated across backends since routing must be identical) and happens to be a non-contiguous view. ggml_backend_meta_buffer_{get,set}_tensor asserted contiguity before consulting the split state, so a mirrored non-contiguous tensor tripped the assert even though the GGML_BACKEND_SPLIT_AXIS_MIRRORED case right below already handles it.

Move the split-state lookup above the assert and allow the mirrored case in both get_tensor and set_tensor.

Diagnosis credit to the reporter (@nathanmp).

Fixes #24886

Signed-off-by: liminfei-amd [email protected]