[dynamic setitem] Fix bug of dynamic setitem: Decerease axes to do right broadcast

[liym27]

PR types

Bug fixes

PR changes

OPs

Describe

Fix bug of setitem when assign a tensor to a tensor.

For example:

x = paddle.ones(shape=[3, 4], dtype="int32")
x[:, 1] = x[:,1] * 2

print(x)

Before this PR,

Tensor(shape=[3, 4], dtype=int32, place=CPUPlace, stop_gradient=True,
       [[1, 2, 1, 1],
        [1, 4, 1, 1],
        [1, 4, 1, 1]])

After this PR,

Tensor(shape=[3, 4], dtype=int32, place=CPUPlace, stop_gradient=True,
       [[1, 2, 1, 1],
        [1, 2, 1, 1],
        [1, 2, 1, 1]])

[paddle-bot-old]

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[zhiqiu]

LGTM