np.linspace 给定起点、终点和项数，返回等差数列
numpy.linspace(start, stop, num=50, endpoint=True, retstep=False)
endpoint 控制终点是否取，retstep 控制是否在第二项返回公差
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print(np.linspace(2.0, 3.0, num=5))
print(np.linspace(2.0, 3.0, num=5, endpoint=False))
print(np.linspace(2.0, 3.0, num=5, retstep=True))
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[2. 2.25 2.5 2.75 3. ]
[2. 2.2 2.4 2.6 2.8]
(array([2. , 2.25, 2.5 , 2.75, 3. ]), 0.25)