F.relu is the functional interface of torch.nn.ReLU. Modules like torch.nn.ReLU are sometimes handy, for example when quickly creating a model using nn.Sequential.
You can’t add F.relu in a nn.Sequential, as it expects an object that is inherited from nn.Module.
About clamp, it is a tensor/variable function which is more generic than ReLU, and it works for both Tensor and Variable, while F.relu only works for variables. Also, ReLU is a common enough module to deserve it’s own function, instead of having to write x.clamp(min=0).