Manipulate multiple of specific network's requires_grad to True or False

I am in trouble in setting up only some specific networks’ requres_grad in my network to True or False.

class MyNetwork:

self.layer1 = Layer1()
self.layer2 = Layer2()
Some other networks exist.

class Layer1():

self.net_a = torch.nn.Linear(xx)
Some other networks exist.

class Layer2():
self.net_b = torch.nn.Linear(yy)
Some other networks exist.

Then I want set up only
to be requires_grad = False.

my_network.parameters() gives iterator, by which you can set up reuires_grad of all networks as follows;

for param in my_network.parameters():
param.requires_grad = True (or False)

however, I can not figure out the way around my_network.parameters() to specify particular network names to be targeted for requires_grad setting.

I investigated my_network.named_modules() but it just returns all subnet names under my_net as an iterator but never gives a linkage to requires_grad setting.
Thank you for your advice in advance.