mb.identity breaks when complex_input.real is None

@LowerComplex.register_lower_func(op_type="complex_real")
def _lower_complex_real(op: Operation):
complex_input: ComplexVar = op.data
# Use an identity op to avoid the block's input name inconsistency issue. If we directly use
# complex_input.real, the var's name could be inconsistent with the block's input name.
breakpoint()
if complex_input.real==None:
breakpoint()
return mb.identity(x=None, before_op=op)
result = mb.identity(x=complex_input.real, before_op=op)
return result

also mb.identity breaks when complex_input.real is None, can we have a workaround for this? @junpeiz

Start at _lower_complex_real and trace the mb.identity call when complex_input.real is None. Reproduce the failure in the model-conversion path and determine a supported workaround for that case; done means conversion no longer breaks.

Written by the indexing model from the issue text.