Function chunk_mask 

pub(crate) fn chunk_mask<B: Backend>(
    chunk_len: usize,
    start: usize,
    valid_len: usize,
    device: &Device<B>,
    dtype: FloatDType,
) -> F<B, 5>

Float validity mask [1, 1, l, 1, 1] for a chunk starting at start (1 for global positions < valid_len, 0 for pads).