fix index_select kernel configuration error where input numel is 0

[FlyingQianMM]

PR types

Bug fixes

PR changes

OPs

Describe

For index_select, if the numel() of input x is 0, launching the kernel raises a error as bellow:

import paddle
data = paddle.empty((0, 7), dtype='float32')
index = paddle.to_tensor([0, 1], dtype='int32')
out = paddle.index_select(data, index, axis=-1)

After we fix this bug, the output can be computed correctly:

[paddle-bot-old]

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[ZzSean]

LGTM