nx_transformer_stack_test.nx

nx_transformer_stack_test.nx -- smoke for nx_transformer_stack.nx. Builds a 1-layer synthetic Llama-shape GGUF with ALL-ZERO weights (so the forward pass becomes an identity), feeds x_in through the stack, and verifies x_in is unchanged. Test spec: hidden=2, head_dim=2, ffn_dim=4, n_tokens=1. Weights = 0 means: - RMSNorm(x, 0) = 0 (gamma=0 zeros out the norm output) - matmul(0, W) = 0 (zero input/weight = zero output) - attention(0, 0, 0) = 0 (softmax of all-zero scores * zero V) - residual: x + 0 = x (UNCHANGED) - FFN: same identity-via-zero pattern So output x == input x after the entire forward pass.