GetExecStream support xpu backend.

[csy0225]

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GetExecStream support xpu backend. // 用户可以在客户端程序中获取 XPUStream，调用 xpu 头文件中 xpu_wait，手动实现 stream 的同步操作。例如：

#include "xpu/runtime.h"
XPUStream xpu_stream = static_cast<XPUStream>(predictor->GetExecStream());
xpu_wait(xpu_stream);

[paddle-bot]

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[hong19860320]

LGTM