fix inconsistent interface implementation

[zhaoyanpeng]

The interface implementation in

pytorch-struct/torch_struct/cky.py

Line 73 in f9f2530

def marginals(self, scores, lengths=None, _autograd=True):

Is inconsistent with its declaration in

pytorch-struct/torch_struct/helpers.py

Line 139 in 876588d

def marginals(self, edge, lengths=None, _autograd=True, _raw=False):

resulting in an error when invoking topk()

pytorch-struct/torch_struct/distributions.py

Lines 96 to 98 in 876588d

	return self._struct(KMaxSemiring(k)).marginals(
	self.log_potentials, self.lengths, _raw=True
	)

This pr will raise an error when invoking unsupported topk() with CKY.

[zhaoyanpeng]

The interface implementation in

pytorch-struct/torch_struct/cky.py

Line 73 in f9f2530

def marginals(self, scores, lengths=None, _autograd=True):

Is inconsistent with its declaration in

pytorch-struct/torch_struct/helpers.py

Line 139 in 876588d

def marginals(self, edge, lengths=None, _autograd=True, _raw=False):

resulting in an error when invoking topk()

pytorch-struct/torch_struct/distributions.py

Lines 96 to 98 in 876588d

	return self._struct(KMaxSemiring(k)).marginals(
	self.log_potentials, self.lengths, _raw=True
	)

This pr will raise an error when invoking unsupported topk() with CKY.

We can now use dist.topk() with CKY.
Hi @srush, could you please review the code?

[srush]

Thanks! CKY is a nasty interface compared to the others, but I wanted to make it fast enough to run.