Training an ensemble model with and without saperately

I have such model. How to train such model because of if condion. I want to optimizer.step with modelA, modelB and modelA+ModelB(MyEnsemble(). Its creating batch issue.

class MyEnsemble(nn.Module):
    def __init__(self, modelA, modelB):
        super(MyEnsemble, self).__init__()
        self.modelA = modelA
        self.modelB = modelB
    def forward(self, x1):
        output = self.modelA(x1)
        if output[0]>0.5:
            out = self.modelB(x1)
        return out

What kind of issue are you seeing? Could you post the complete error message and, if possible, extend the code snippet to reproduce this issue?