Remove TensorConstant.tag.unique_value

[brandonwillard]

TensorConstant.tag.unique_value is an unnecessary specialized field used in only a few places. This field also overlaps with the logic in get_scalar_constant_value and extract_constant, making for yet another redundancy. Since this redundancy is attached to a core type, its removal has some priority over the other two functions.

In general, it can be easily replaced by a simple helper function—one that could also replace some of the functionality provided by get_scalar_constant_value and extract_constant, both of which are also slated for removal/replacement (see here).

[Carlosbogo]

I would be glad to help with this, but I think it would be very helpful to have a more in depth explanation on what the term "helper function" implies here.

[brandonwillard]

Constants, and TensorConstants by extension, hold their constant values (i.e. np.ndarrays) in a Constant.data field. TensorConstant.tag.unique_value is supposed to be a scalar "reduction" of that field, so, for instance, if the field's value is a single scalar value duplicated/broadcasted in some way, TensorConstant.tag.unique_value will be that scalar value.

This logic is specified here, and it can be extracted into a simple helper function with a name like get_unique_value and used explicitly by any logic currently depending on TensorConstant.tag.unique_value.

From what I can see, these are the only references to unique_value in the project:

./aesara/scan/opt.py:169:            and node_inp.tag.unique_value is not None
./aesara/tensor/math_opt.py:135:        if getattr(v.tag, "unique_value", None) is not None:
./aesara/tensor/math_opt.py:136:            data = v.tag.unique_value
./aesara/tensor/var.py:969:        self.tag.unique_value = None
./aesara/tensor/var.py:974:                    self.tag.unique_value = flat_data[0]
./aesara/tensor/var.py:977:        if self.tag.unique_value is not None:
./aesara/tensor/var.py:978:            name = f"{self.data.shape} of {self.tag.unique_value}"
./aesara/tensor/basic.py:285:        if getattr(v.tag, "unique_value", None) is not None:
./aesara/tensor/basic.py:286:            data = v.tag.unique_value
./aesara/tensor/basic.py:299:        if getattr(v.tag, "unique_value", None) is not None:
./aesara/tensor/basic.py:300:            data = v.tag.unique_value