ops: transformer op fwd/bwd CUDA kernels + Tensor wrappers

add/mul/add_bias(+sum_rows)/rms_norm/silu/rope/softmax/cross_entropy,
each with its analytic backward, in csrc/ops/nn.cu (inlined warp/block
reductions). FFI declarations + nn.cu in build.rs (no_cuda gated). Tensor
gains the matching thin wrappers; DType grows I32 for cross-entropy targets.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-cuda/build.rs

@@ -32,6 +32,7 @@ fn main() {
         .file("../../csrc/test/vecadd.cu")
         .file("../../csrc/ops/elementwise.cu")
         .file("../../csrc/ops/gemm.cu")
+        .file("../../csrc/ops/nn.cu")
         .compile("xtrain_cuda_kernels");
 }