SGD with Momentum

Implement a single SGD with momentum parameter update.

Signature: def sgd_momentum_step(theta: np.ndarray, grad: np.ndarray, v: np.ndarray, lr: float = 0.01, mu: float = 0.9) -> np.ndarray

Return the updated theta_new.

• Velocity update: v = mu*v - lr*grad
• Parameter update: theta += v