Yes, my fault. The following should work,
weight_to_be_used = []
for mod in net.features.children():
    if isinstance(mod, nn.modules.conv.Conv2d)
    weight_to_be_used.append(mod.weight.data)


mod is all the modules in net.features, convolution modules has two parameters: weight and bias, which can be accessed by mod.weight and mod.bias attribute. The underlying tensor can be accessed using data attribute.