Device mismatch in LinearChainCRF

[kmkurn]

This line below causes device mismatch error if chart is in GPU because init will always be created in CPU.

pytorch-struct/torch_struct/linearchain.py

Line 56 in e51fecc

init = torch.zeros(*chart.shape).bool()

I think the fix would be torch.zeros_like(chart).bool().

[srush]

Thanks. Need to get the gpu tests in the CI.