bug描述 Describe the Bug

python 通信API: alltoall_single 文档中声明了 in_split_sizes 的长度只要可以被 world_size 整除即可，然而在后端实现中只支持 in_split_sizes==world_size 的情况，这一点与文档不符合，同时也无法对齐 torch 的API。

以下是对应的后端代码：
https://github.com/PaddlePaddle/Paddle/blob/incubate/new_frl/paddle/fluid/distributed/collective/process_group_nccl.cc#L250

运行报错：

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Error Message Summary:
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InvalidArgumentError: The length of size_on_each_rank must be equal to world_size.
  [Hint: Expected length_size_on_each_rank == world_size, but received length_size_on_each_rank:16 != world_size:8.] (at /work/Paddle/paddle/fluid/distributed/collective/process_group_nccl.cc:222)

其他补充信息 Additional Supplementary Information

No response