If you have an input of shape (T, B), you will get a packed sequence with shape (sum of lengths,) as data.
If you have an input of shape (T, B, F_1), you will get a packed sequence with shape (sum of lengths, F_1) as data.
If you have an input of shape (T, B, F_1, F_2), you will get a packed sequence with shape (sum of lengths, F_1, F_2) as data.
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So * is a wildcard (similar to its use in file glob masks) representing any possible value of input.shape[2:].
Best regards
Thomas