Switch to `nan_to_num` (rm `where`) in the future

[Saransh-cpp]

#595 adds a workaround for #593's original CuPy issue - cupy/cupy#9143. The workaround includes using where instead of nan_to_num in specific compute functions.

Using where is okay for numerical backends, but it is an issue for the SymPy backend. The shim _lib.where function is not a generic replacement for np.where, instead, it works only for this particular case (return the second argument (first value) as we know that the "normal" values are in the second argument and the "exceptional" ones are in the third argument). Therefore, this occurrence of where is safe to use, but should not be used out of "context" / as a drop-in replacement for np.where. It would be nice to remove the occurrences of where from compute functions (revert #595) once the CuPy issue is fixed (so that the compute functions stay generic).

[ikrommyd]

PR is merged in cupy: cupy/cupy#9240
It's targeted for the 14.0.0 release. We can undo this some time later.