Fix list slice & ConstantVariable to TupleVariable conversion missing source info

[yanboliang]

This is to resubmit #222.
Fix #231, now ./torchbench.py -k yolov3 works well.

• Root cause: ConstantVariable.call_method wraps value as TupleVariable and call its call_method, but doesn't propagate source info in this conversion, so the source chain is missed when codegen.
• Why torchbench doesn't fail before Fix slicing list returning wrong result #222?
  • Because it falls into another branch here previously. Actually I think we can switch the order to first check python constant:

if value.is_python_constant() and is_safe_constant(
   value.as_python_constant()
):
   output.append(self.create_load_const(value.as_python_constant()))
elif value.source is not None and allow_cache:
   output.extend(value.source.reconstruct(self))

As this can generate more concise code if the stack value is python constant compatible(only LOAD CONST in this case, and doesn't need to trace the source). I'd like to know if this is aligned with the design philosophy and update this PR if it makes sense.