Add Flatten, and take batch norm's statistics per channel - #101
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On a 4-D activation the old reduction gave every pixel position its own mean and variance, normalizing each position against the batch rather than normalizing the channel.
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A conv stack had no way to reach a dense head, and batch norm placed after a convolution was computing the wrong thing.
Flattencollapses everything after the batch axis. Batch norm now reduces over every axis but the last, so a channel gets one mean and variance instead of every pixel position getting its own against the batch. It also rejects rank-1 input now, which has no batch axis to take statistics over and used to normalize across features instead.BatchNorm2Dis renamed toBatchNorm, since the reduction rule is identical at every rank. That's what flax and mlx each do with a single class; torch's1d/2d/3dsplit is three rank validators over one shared rule rather than three semantics.