[bug] Fix name collision in ti.dataclass

[strongoier]

Issue: fix #6652

Brief Summary

Properties should be added to a certain Struct instance instead of the Struct class itself.

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[ailzhang]

Thanks!

[YouJiacheng]

Use __getattr__ and __setattr__ might be a better choice than create a new class for each instance.
And that will break user's code like (fortunately, taichi use isinstance internally)

foo = ti.Struct(...)
if type(foo) is ti.Struct:
    # do something
    pass

Use __getattr__ and __setattr__ can remove boilerplates _make_getter and _make_setter as well. (BTW, why setter requires python scope while __setitem__ doesn't?)

If setter has the same requirements as __setitem__:

class Struct(TaichiOperations):
    # remove _register_members, _make_getter and _make_setter
    def __getattr__(self, name):
        try:
            return self[name]
        except KeyError:
            raise AttributeError(f"'{self.__class__.__name__}' object has no attribute '{name}'") from None
     def __setattr__(self, name, value):
         if name not in self.entries:
             return super().__setattr__(name, value)
         self[name] = value

This implementation has an additional benefit: raise correct Error(AttributeError instead of KeyError) after entries modified by users.(entries is a public field thus subject to change)

[strongoier]

Sounds good! @YouJiacheng Are you interested in sending a PR for this?

(BTW, why setter requires python scope while setitem doesn't?)

Because in taichi scope __setitem__ will never be called. The assignment will be handled by the AST transformer.

[YouJiacheng]

Hmmm, IIUC attribute assignment will be handled by the AST transformer as well. I will send a PR for this after my final exam(2023.1.4).