[0-size Retest] Fix var、std、selu api to support 0-size Tensor

[DanielSun11]

PR Category

Operator Mechanism

PR Types

Bug fixes

Description

修复复测发现的0-size问题

• var相关的api（var、std）grad为none

def _replace_nan(out):
        out_nan = paddle.full_like(out, paddle.nan)
        out_nan.stop_gradient = out.stop_gradient
        return out_nan 

full_like没有反向，导致梯度回传时回传不到输入x

• selu反向过程报cuda error 9

修复完paddleapitest回测
var

selu

pcard-67164

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Additional details and impacted files

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  Files              ?         1           
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  Hits               ?         2           
  Misses             ?         0           
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