You can create sub modules of the model and train them in multi devices
class MyModel(nn.Module):
    def __init__(self, split_gpus):
        self.large_submodule1 = ...
        self.large_submodule2 = ...

        self.split_gpus = split_gpus
        if split_gpus:
            self.large_submodule1.cuda(0)
            self.large_submodule1.cuda(1)

    def forward(self, x):
        x = self.large_submodule1(x)
        if split_gpus:
            x = x.cuda(1) # P2P GPU transfer
        return self.large_submodule2(x)