Lazy imports in detectron2

Summary: This should reduce most of the import time for `detectron2.engine` (which is about 0.7s currently, see https://fburl.com/scuba/import_prof/6tywe8xm)

Reviewed By: ppwwyyxx

Differential Revision: D26072937

fbshipit-source-id: d34aae9c974f4a3d8ec97595ab6f216094d86f15

Author: danthe3rd

detectron2/utils/events.py

@@ -6,7 +6,6 @@
 import time
 from collections import defaultdict
 from contextlib import contextmanager
-import torch
 from fvcore.common.history_buffer import HistoryBuffer
 
 from detectron2.utils.file_io import PathManager
@@ -198,6 +197,8 @@ def __init__(self, max_iter):
         self._last_write = None
 
     def write(self):
+        import torch
+
         storage = get_event_storage()
         iteration = storage.iter
         if iteration == self._max_iter:
@@ -343,6 +344,8 @@ def put_histogram(self, hist_name, hist_tensor, bins=1000):
                 into a histogram.
             bins (int): Number of histogram bins.
         """
+        import torch
+
         ht_min, ht_max = hist_tensor.min().item(), hist_tensor.max().item()
 
         # Create a histogram with PyTorch