What is the meaning of padding-free in ReadMe?

[houghtonweihu]

In the readme, it says:

The OpenChat training system utilizes padding-free training and the Multipack Sampler, achieving a 3~10x speedup compared to the conventional padded training.

What is the meaning of padding-free here? Is there a need for all seqs in one batch
to have the same length? If no padding, how is this done?

Thanks!