Fix plan stream in eagle worker v2

[silencejade]

Motivation

When using the Eagle Worker V2, the prepare_for_v2_verify function relies on batch.spec_info.positions to compute next_input_positions on the planning stream. However, batch.spec_info.positions is created and calculated within the build_tree_kernel_efficient function, which executes on the main stream. This results in a synchronization issue between the two streams.

Modifications

Insert a synchronization event for build_tree_kernel_efficient prior to executing prepare_for_v2_verify.

Accuracy Tests

Speed Tests and Profiling

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