How to understand this block of python code?

Question

Found a piece of python code that can run perfectly,but I couldn't understand how it works. Would appreciate it if you could explain the lining part for me. I totally don't know what it does

class BoxHead(nn.Module):#pending
    def __init__(self, lengths, num_classes):
        super(BoxHead, self).__init__()
#-------------------------------------------------------
        self.cls_score = nn.Sequential(*tuple([
            module for i in range(len(lengths) - 1)
            for module in (nn.Linear(lengths[i], lengths[i + 1]), nn.ReLU())]
            + [nn.Linear(lengths[-1], num_classes)]))
#-----------------------------------------------------------------------

        self.bbox_pred = nn.Sequential(*tuple([
            module for i in range(len(lengths) - 1)
            for module in (nn.Linear(lengths[i], lengths[i + 1]), nn.ReLU())]
            + [nn.Linear(lengths[-1], 4)]))

Answer 1

After reading, I assert lengths as a list of numbers.

This is the input of nn.Sequential for cls_score:

# lengths = [num1, num2, num3, ..., numN-1, numN]
# numN-1 just represents its meaning, and is not valid in python syntax
[
    nn.Linear(num1, num2),
    nn.ReLU(),
    nn.Linear(num2, num3),
    nn.ReLU(),
    nn.Linear(num3, num4),
    nn.ReLU(),
    ...,
    nn.Linear(numN-1, numN), 
    nn.ReLU(),
    nn.Linear(numN, num_classes)
]

Input of nn.Sequential for bbox_pred is alike, but its last item is nn.Linear(numN, 4)