ランダムテンソル (3x3):
[[0.5234 0.1823 0.7412]
 [0.3091 0.8834 0.2219]
 [0.6701 0.4412 0.1983]]
行列積 x @ x.T:
[[0.8631 0.4312 0.5801]
 [0.4312 0.9321 0.4108]
 [0.5801 0.4108 0.6691]]
autograd: x=3, y=x^2+2x+1, dy/dx = 8.0 (期待値: 8.0)