[TVMScript] Produce empty DictAttrs when R.func_attrs is absent

[Lunderberg]

A follow-up to #16745. For Relax functions produced in TVMScript, when R.func_attrs was not present, the default was set to None instead of an empty dictionary.

[slyubomirsky]

What is the specific reason to have this change? It might be a good invariant to have, just wondering if there was a specific motivation for it.

[Lunderberg]

Overall, to remove a repeated source of bugs. Since most functions are publicly exposed, they'll have at least the "global_symbol" attribute. Frequently, utilities would be written that only worked for functions that had non-empty attributes. For example, checking if 'my_attribute' in func.attrs instead of if func.attrs is not None and 'my_attribute' in func.attrs. These instances could look reasonable for quite some time, until a function that happens to have no attributes would trigger the bug.

This class of bug was only possible because we had two entirely separate ways to represent "has no attributes". Changing the default from None to an empty DictAttrs removes the opportunity for these bugs to occur.

[slyubomirsky]

I think I support your reasoning, this is an easily averted source of errors.