The error is raised, as norm cannot be expanded to masked_embedding:
masked_embedding = torch.randn(256, 66, 64)
norm = torch.randn(256, 66)
norm.expand_as(masked_embedding)
> RuntimeError: The expanded size of the tensor (64) must match the existing size (66) at non-singleton dimension 2.  Target sizes: [256, 66, 64].  Tensor sizes: [256, 66]

so you might want to add the missing dimension:
masked_embedding = torch.randn(256, 66, 64)
norm = torch.randn(256, 66).unsqueeze(2)
norm.expand_as(masked_embedding)