[serialization_lib] update tosa_serializer.py

Adds properties "is_unranked", "is_variable" and "variable_name" to
TosaSerializerTensor in tosa_serializer.py. these properties have
been missing from the python api.

Also added tests in tosa_single_tensor.py for (1) unranked and (2)
variable intermediate tensors

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: Ie9d5da0e26eb401e7663a8fa91d563e1d5786855