[CodeStyle][Xdoctest][17,21,27,28] Fix example code(`paddle.Tensor.matmul`,`paddle.Tensor.new_empty`,`paddle.Tensor.sgn`,`paddle.Tensor.shape`,)

[WHoutstanding]

PR Category

User Experience

PR Types

Others

Description

为如下 API 修复示例代码

• paddle.Tensor.matmul
• paddle.Tensor.new_empty
• paddle.Tensor.sgn
• paddle.Tensor.shape

Related links

• #76545

@ooooo-create

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