It depends how you construct the optimizer.
If you do
optimizer = optim.SGD(model.parameters(), lr = 0.01, momentum=0.9)

that means you only have one param group.
If you do
optim.SGD([
                {'params': model.base.parameters()},
                {'params': model.classifier.parameters(), 'lr': 1e-3}
            ], lr=1e-2, momentum=0.9)

that means you have two param groups.
Sorry, I edited the first post. It seems that changing .defaults won’t change the optimizer setup for param group 0 (when there is only one group).
Sorry, I should have searched enough before answering the question.