This commit resolves a bug that was introduced in
https://github.com/apache/tvm/pull/16377. If no CUDA-capable GPUs are
present, the call to `cudaGetDeviceCount` will return an error, which will
be raised as an exception by the `CUDA_CALL` macro. However, checking
the `kExist` flag is valid even if no GPUs are present.
This commit removes the use of `CUDA_CALL`, and instead returns false
in this case.
Eric Lunderberg