Change BaseTrack attributes to Object attributes

All the attributes are used after  object initialisation only, so there is no reason to keep them as Class attributes.  
Keeping them as class attributes instead causes ifzhang#79 

This should fix ifzhang#79 and should not affect any other part.  
Have checked that we always call `self.next_id()` and never call `BaseTrack.next_id()` i.e., no need for it to be a static method anymore, since all the attributes are now object attributes.

yolox/tracker/basetrack.py

@@ -10,31 +10,31 @@ class TrackState(object):
 
 
 class BaseTrack(object):
-    _count = 0
+    def __init__(self):
+        self._count = 0
 
-    track_id = 0
-    is_activated = False
-    state = TrackState.New
+        self.track_id = 0
+        self.is_activated = False
+        self.state = TrackState.New
 
-    history = OrderedDict()
-    features = []
-    curr_feature = None
-    score = 0
-    start_frame = 0
-    frame_id = 0
-    time_since_update = 0
+        self.history = OrderedDict()
+        self.features = []
+        self.curr_feature = None
+        self.score = 0
+        self.start_frame = 0
+        self.frame_id = 0
+        self.time_since_update = 0
 
-    # multi-camera
-    location = (np.inf, np.inf)
+        # multi-camera
+        self.location = (np.inf, np.inf)
 
     @property
     def end_frame(self):
         return self.frame_id
 
-    @staticmethod
-    def next_id():
-        BaseTrack._count += 1
-        return BaseTrack._count
+    def next_id(self):
+        self._count += 1
+        return self._count
 
     def activate(self, *args):
         raise NotImplementedError
@@ -49,4 +49,4 @@ def mark_lost(self):
         self.state = TrackState.Lost
 
     def mark_removed(self):
-        self.state = TrackState.Removed
+        self.state = TrackState.Removed