x[b].transpose(1, 0)[b].transpose(1, 0)
should do the trick.
Strange that the error message to x[b, :] reads:

TypeError: indexing a tensor with an object of type torch.LongTensor. The only supported types are integers, slices, numpy scalars and torch.LongTensor or torch.ByteTensor as the only argument.

Which seems self-contradictory