add display level to htrace_to_chrome.py
add display level to htrace_to_chrome.py

Summary: This diff adds an option to the htrace_to_chrome.py format conversion script so that users can decide to display less traces by hiding kernel/operator/worker spans. For example, passing the arguments `--display worker` will make the script process spans up to worker spans and not go further (deeper).

Differential Revision: D4360404

fbshipit-source-id: aa5af7e499b94aeb3de06823bdeeedfbc3b1c02b

sf-wind/caffe2,caffe2/caffe2,bwasti/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,davinwang/caffe2,xzturn/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,bwasti/caffe2,bwasti/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,sf-wind/caffe2,davinwang/caffe2,bwasti/caffe2,pietern/caffe2,pietern/caffe2,Yangqing/caffe2,bwasti/caffe2,pietern/caffe2