[cherry-pick] fix flatten infershape

[danleifeng]

PR types

Bug fixes

PR changes

OPs

Describe

cherry pick #35321

fix flatten op infershape

ori:

>>> x_var = paddle.static.data(name="y", shape=[None,None,1], dtype='float32')
>>> out = paddle.flatten(x_var)
>>> print(out.shape)
(1,)

fix after:

>>> x_var = paddle.static.data(name="y", shape=[None,None,1], dtype='float32')
>>> out = paddle.flatten(x_var)
>>> print(out.shape)
(-1,)

[paddle-bot-old]

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[yaoxuefeng6]

LGTM