Remove boost::variant for FetchResultType

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Describe

为绕开PR #43100 中遇到的将FetchResultType由boost::variant替换成paddle::variant后触发Windows平台installation_validate错误问题，将ParalleExecutor的run接口改造成不返回FetchResultType。
改造前：

using FetchResultType = boost::variant<FetchList, FetchUnmergedList>;
FetchResultType Run(const std::vector<std::string> &fetch_tensors,
                      bool return_merged = true);

改造后：

FetchUnmergedList Run(const std::vector<std::string> &fetch_tensors);
FetchList RunAndMerge(const std::vector<std::string> &fetch_tensors);

同时将FetchResultType由boost::variant替换成paddle::variant。

[paddle-bot-old]

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[zhiqiu]

LGTM