Refactor cat and broadcast_tensors error handling.

[ysiraichi]

This PR refactors the implementation of cat and broadcast_tensors in the tensor_methods.cpp file.
In summary:

• Make both of them call CheckNonEmptyInputs, returning an error status on failure
• Factor the other cat() error into its own function: CheckCatCompatibleShapes
• Make broadcast_tensors use status constructs

Note that there are no tests for these changes because:

• cat() is already tested
• broadcast_tensors() is used only internally