Comment in `StructuredDot.grad` is not correct

[tomicapretto]

Description

The following comment is not correct:

pytensor/pytensor/sparse/math.py

Lines 1396 to 1402 in 18fbf45

	def grad(self, inputs, gout):
	# a is sparse, b is dense, g_out is dense
	# ga = g_out x b.T
	# gb = a.T x g_out
	(a, b) = inputs
	(g_out,) = gout
	return [structured_dot_grad(a, b, g_out), structured_dot(a.T, g_out)]

because b and g_out could be sparse, too.

However, they are not supported when performing the operation via C code, only via Python (and soon via numba as well).