midman-tensor
What it does: dense f32 tensors on the CPU with reverse-mode automatic differentiation, every operation a transformer needs, and a finite-difference gradient checker.
Example: build x.matmul(&w).silu().sum_all(), call backward(), and read the gradient of w.
Status: done and tested. Single-threaded; broadcast and softmax are measurably slower than a plain loop (see midman-bench).