also my question is in the context of padding not packing…so idk why packing is relevant…
https://pytorch.org/docs/stable/nn.html#pad-sequence 2

pad_sequence
torch.nn.utils.rnn.pad_sequence(sequences, batch_first=False, padding_value=0)[SOURCE]
Pad a list of variable length Tensors with padding_value
pad_sequence stacks a list of Tensors along a new dimension, and pads them to equal length. For example, if the input is list of sequences with size L x * and if batch_first is False, and T x B x * otherwise.
B is batch size. It is equal to the number of elements in sequences. T is length of the longest sequence. L is length of the sequence. * is any number of trailing dimensions, including none.