AutoTVM log format

[haichen]

@comaniac

4. I raised this issue in the op strategy PR but I think this is a great place to revisit it: The op strategy PR now leverages the measured time to determine which implement should be applied when compilation. To do so, it now adds the cost to input when loading logs, because we will pass result to the Relay build flow. According to this requirement, should we consider changing the log format to a form like the following:

Just to provide more context about this. Relay op strategy needs to compare the performance between different implementation and therefore requires to get the cost from autotvm log. The way to achieve this currently is that I add the cost field in the config and load the cost from autotvm log.