ops: embedding/reshape/transpose/split-merge-heads fwd+bwd

Phase T5 structural ops on top of the T4 set, needed to assemble the
tiny transformer:
- embedding: gather rows by I32 ids (CUDA kernel) / scatter-add backward
  (atomic, so repeated ids accumulate). csrc/ops/model.cu + ffi.
- reshape: contiguous metadata-only view (Tensor::reshape), no kernel.
- transpose_3d01: [a,b,c]->[b,a,c] for the multi-head layout (kernel).
- autograd nodes: embedding/reshape/transpose_3d01/transpose_2d, plus
  split_heads (->Vec<Var>) / merge_heads for per-head attention.
- tape: Var::zero_grad + set_value so a hand-written GD step can update
  params and clear grads between steps.

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

crates/xtrain-cuda/build.rs

@@ -33,6 +33,7 @@ fn main() {
         .file("../../csrc/ops/elementwise.cu")
         .file("../../csrc/ops/gemm.cu")
         .file("../../csrc/ops/nn.cu")
+        .file("../../csrc/ops/model.cu")
         .compile("xtrain_cuda_kernels");
 }