Created by: zhiqiu

As the title, add FLAGS_imperative_check_nan_inf to control NAN/INF check in imperative mode.

FLAGS_imperative_check_nan_inf = 0, disable imperative NAN/INF check. FLAGS_imperative_check_nan_inf = 1, enable imperative NAN/INF check, raise exception and stop execution when NAN/INF detected at the first time. FLAGS_imperative_check_nan_inf = 2(>1), enable imperative NAN/INF check, do not raise exception but log the message and continue execution.