Sequential can only be a collection of Module (or a dict where the values are Module). So you can wrap your reshape op in a module like below and include it in features.
class ReshapeModule(nn.Module):
     def forward(self, x):
         x = x.view(x.size(0), -1)
         return x

features = nn.Sequential(
        nn.Conv2d(3, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1)),
        nn.ReLU(inplace=True),
        ...
        nn.MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False),
        ReshapeModule(),
        nn.Dropout(p=0.5, inplace=False),
        ...
)