Fix H2D stream synchronization in optimizer offload

[tgkyrie]

In param_copy_back_gpu_hook, the H2D memcpy is enqueued on _h2d_stream, but the previous code incorrectly recorded an event on _d2h_stream and made the main stream wait on it. This does not guarantee the main stream waits for the H2D copy to complete, which can lead to race conditions.

This commit records the event on _h2d_stream instead, ensuring proper happens-before semantics between H2D memcpy and subsequent GPU operations.

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[dimapihtar]

LGTM. Thank you!

[dimapihtar]

/ok to test 5430b04

[yaox12]

/ok to test c4d772c