Support zero value in dimension for slice

[zyfncg]

PR types

New features

PR changes

OPs

Describe

Slice索引增加了对切片部分维度为0的功能支持，即允许切片索引结果为空（之前会进行检测并报错）。

Example:

Before this PR:

>>> x = paddle.to_tensor([[1, 2, 3], [4, 5, 6]])
>>> print(x[:, 0:0])
#  ValueError: (InvalidArgument) When step > 0, end should be greater than start, but received end = 0, start = 0.
#  [Hint: Expected end > start, but received end:0 <= start:0.]

Now:

>>> x = paddle.to_tensor([[1, 2, 3], [4, 5, 6]])
>>> print(x[:, 0:0])
# Tensor(shape=[2, 0], dtype=int64, place=CUDAPlace(0), stop_gradient=True,
#              [[],
#               []])

[paddle-bot-old]

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

[chenwhql]

LGTM

[MingMingShangTian]

LGTM