Add querying OpenCL device infomation

[marty1885]

Hi, this pull request allows TVM to have the correct warp size and thread count when using OpenCL.
This PR is not compatible with some parts of #1290. But I believe this is a better approach that it provides accurate information about the device on all devices.

[tqchen]

thanks for proposing this. The warp information was only used to perform warp level optimization(in opencl's term subgroup memory). So far it is only enabled for intel graphics device, where we know the necessary multiple. A better way to get this value, than querying by dummy kernel, might be check if the subgroup extension is available and query subgroup size.

On the other hand, we cannot directly do runtime query during of target creation. This is mainly because that the compiler may exist in places other than the target device(e.g. compile on your laptop, runs on your phone), and the on the fly query can be inaccurate.

[tqchen]

So to summarize, we might need to keep the old conservative way of warp size assignment in target creation, but maybe could enhance opencl runtime a bit to also query return a correct warp size(using subgroup info query API).

[tqchen]

@marty1885 can you update the PR according to the feedbacks?

[tqchen]

close for now due to inactive status, let us open another PR based on the discussion