Ensure the process id to thread id to node id index is maintained. (#200)

Summary:

Currently this map is not set since __ROOT_THREAD__ is not a valid node anymore. We can safely read thr pid and tid against a node id and store in this map.

Reviewed By: shengfukevin

Differential Revision: D71565563

Author: sanrise

et_replay/execution_trace.py

@@ -357,8 +357,8 @@ def __init__(self, json):
             input_tensors = self.nodes[id].get_input_tensors()
             output_tensors = self.nodes[id].get_output_tensors()
 
-            # track the various process and threads we have
-            if x["name"] == "__ROOT_THREAD__":
+            # track annonation to get thread ids of root nodes
+            if x["name"] == "[pytorch|profiler|execution_trace|thread]":
                 tid = self.nodes[id].tid
                 self.proc_group[pid][tid] = id