Missing GPU implementation for Trace

With commit c5dd9c4be2bc4d935434e048002f772e602b24cb

When I try to perform trace for a UniTensor, I got RuntimeError
````
RuntimeError: 
# Cytnx error occur at cytnx::Tensor cytnx::linalg::Trace(const cytnx::Tensor&, const cytnx::cytnx_uint64&, const cytnx::cytnx_uint64&)
# error: [ERROR][Trace] GPU is under developing.

# file : /home/pcchen/github/Cytnx-dev/Cytnx/src/linalg/Trace.cpp (91)
````