Add fold opereators

[tink2123]

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add fold opereators

import paddle.nn as nn
import paddle
data = paddle.randn([2,3*2*2,12]).astype("float32")
fold = nn.Fold(output_sizes=(4, 5), kernel_sizes=2)
output = fold(data)
# output.shape = [2,3,4,5]

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[jeff41404]

LGTM