b10751

<details open>

cuda: fuse MoE weighted expert reduction (#25952)

* cuda : fuse MoE weighted reduction (mul + view + add)

The MoE combine tail currently writes weighted expert outputs to
global memory before reducing them. That intermediate global-memory
traffic is the main cost. The production baseline generally runs two
physical fused kernels; this path runs one.

This change matches the full expert-weighting plus ordered-reduction
subgraph and replaces it with one weighted-reduction kernel.

Supported graphs:
- unscaled: experts * router_weights
- scaled:   (experts * expert_scale) * router_weights

k = 2..15 is handled by one runtime-k kernel.

Matching is structural: op sequence, shapes, strides, expert views,
and the left-to-right ADD chain. The fused kernel keeps that same
reduction order. Results are not claimed bit-identical; CUDA FP32
contraction can change rounding slightly.

Allocator integration uses add_alloc_dep from the graph-optimizer
API so experts, router weights, and optional expert scales stay live
until the fused destination is written. Memory ranges are rechecked
before the fused kernel runs.

Unrecognized or unsafe graphs are left alone and keep the existing
per-op path. Set GGML_CUDA_MOE_WEIGHTED_REDUCTION=0 to disable the
fusion.

test-backend-ops covers scaled/unscaled, aligned/unaligned, and
representative values across k=2..15, plus a k=16 case that must
stay on the per-op path.

* Pruned the test matrix from 15 to 6

* Addressed the aman and olivers review comments

</details>

**Website:**
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**Attestations:**
- <https://github.com/ggml-org/llama.cpp/attestations/44516498>

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