Fixed left_pad_sequence - correctly flip dims based on batch_first

[mirceamironenco]

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Changelog

• Fix left_pad_sequence (from data/_collate.py) to have feature parity with pad_sequence from torch.nn.utils.rnn. Previous version would not account for the case where batch_first is False when post-flipping the padded sequence.
• Added corresponding test.

Note that (I believe) future versions of pytorch (> 2.4.1) will expose a padding_side argument which could replace this utility altogether.

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[RdoubleA]

Thanks for the fix!

[SalmanMohammadi]

Could I be annoying and ask if you could explicitly write out the expected tensor, please? Helps a lot to understand what's going at a glance.

[mirceamironenco]

Sure! I've added the expected tensor for the batch_first=False case.