The error is a shape mismatch as returned a bit higher in the stack trace:
RuntimeError: mat1 and mat2 shapes cannot be multiplied (64x2 and 4096x4096)

This is caused because you are assigning the new nn.Sequential block to model.classifier[0] and keep all other modules from the original model.classifier (i.e. model.classifier[1:7]).
I guess you want to replace the entire classifier with the new nn.Sequential block, so use:
input_size = model.classifier[0].in_features    
model.classifier = nn.Sequential(
    nn.Linear(input_size, 128), nn.ReLU(),
    nn.Linear(128, 2))