Fix errors which caused some unit tests failed to compile.

[ZJUGuoShuai]

MatX sets -Werror in its main CMakeLists.txt:

set(WARN_FLAGS ${WARN_FLAGS} $<$<COMPILE_LANGUAGE:CUDA>:-Werror all-warnings>)
set(WARN_FLAGS ${WARN_FLAGS} $<$<COMPILE_LANGUAGE:CXX>:-Werror>)

Thus all warnings are treated as errors, which may lead to compilation failures.

In this PR, I fixed some errors (warnings, e.g. comparing signed/unsigned numbers) and now all unit tests compiles fine.

[cliffburdick]

/blossom-ci

[cliffburdick]

Thanks @AtomicVar!