[TOPI] Fix CUDA Library Tuning

[comaniac]

Although we make conv2d_cudnn.cuda and dense_cublas.cuda as AutoTVM tasks so that they can be "tuned" and compared with other implementations, there have some issues prevent us from actually "tuning" them.

• conv2d_cudnn.cuda: I constantly got the following errors in cudnnFindConvolutionForwardAlgorithm (on T4 GPU). Note that this function is called when extracting tasks without issues. I guess it might be the issue of CUDA context and threading.

  cuDNN: Check failed: e == CUDNN_STATUS_SUCCESS (2 vs. 0) : CUDNN_STATUS_ALLOC_FAILED
  

  The solution in this PR is to create a knob so that it becomes a template with 8 candidates. In this case, cudnnFindConvolutionForwardAlgorithm will not be called during tuning and everything works well. We still set the knob value to -1 in the fallback config to achieve the same behavoir as now.

• dense_cblas.cuda: The error comes from the callback function that tries to display the FLOPS. The reason is that task.flops is FloatImm instead of float, so float(flops) will throw type error. This PR lets add_flop function support FloatImm and IntImm types.

cc @icemelon9 @merrymercy