cuda : add BF16 support for XIELU (#29955)
The XIELU CUDA kernel template is already generic over the element
type; only the F32/F16 type assertion and the else-if dispatch were
missing. Add the nv_bfloat16 branch to the launcher, and drop the
temporary supports_op gate in ggml-cuda.cu that rejected BF16+XIELU.
test-backend-ops gains two BF16 cases ([10,5,4,3] and [512,16,1,1]).
docs/ops/CUDA.csv and docs/ops.md are regenerated; the F32 xIELU row
flips from no to yes as well, i.e. the previous record was stale.
Tested:
- Mac CPU: xIELU F32/F16/BF16, 6/6
- Mac Metal: existing F32/F16, 4/4; BF16 still unsupported
- RTX 4090 CUDA: xIELU F32/F16/BF16, 6/6
- RTX 4090 CUDA BF16-only: 2/2
- git diff --check passes
