[feat] Add PyNvBatchAsyncStreamReader for 2D batched async stream dec…

[xupinjie]

Description

Introduces a new async-only video decoder under accvlab.on_demand_video_decoder that decodes V videos × F frames per video in a single in-flight async submission, returning List[List[RGBFrame]] indexed [v][f]. Targeted at StreamPETR-like workloads consuming multi-sweep batches per training step.

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