Arm backend: Convert assert to throw ValueError in op_sigmoid (#9875)

Asserts are converted to proper raises to ensure graph integrity.

It should not be possible for sigmoid to have more than 1 input for a
correctly formatted graph, but in the node visitor we cannot know for
sure that the graph is formatted correctly.

torch.sigmoid supports more data types than fp32, which is why it should
be checked.

Signed-off-by: Sebastian Larsson <sebastian.larsson@arm.com>