Support batching in point attention v3 battery

[swahtz]

📋 Originally created by @fwilliams

Original author: @fwilliams
Date: 2025-08-07T17:52:10Z

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Currently point attention network works on batch size 1, we should support arbitrary batch size and compare with the baseline ptv3 in the test.

[fwilliams]

This should be in fvdb-examples

[fwilliams]

@blackencino this is done right?