return_err for Svd_truncate

[pcchen]

Currently, when the input of cytnx.linalg.Svd_truncate() is a UniTensor and return_err=True, it will return return_err as a UniTensor.

I think we should just return return_err as a scalar.

[hunghaoti]

This problem caused from the constrain of C++. C++ can only one type variable like:

std::vector<cytnx::UniTensor> Svd_truncate(...) {
...
}

not like python can return several type:

svds, error = Svd_truncate(...)

Since the library use pybind so we just put the error info in the UniTesnor.