[Fix]: fix deadlock in multi-process evaluation (#52)

* debug for issue #45

* push for lint

mmrotate/datasets/dota.py

@@ -8,7 +8,7 @@
 import zipfile
 from collections import defaultdict
 from functools import partial
-from multiprocessing import Pool
+from multiprocessing import get_context
 
 import mmcv
 import numpy as np
@@ -380,7 +380,7 @@ def eval_map(det_results,
     area_ranges = ([(rg[0]**2, rg[1]**2) for rg in scale_ranges]
                    if scale_ranges is not None else None)
 
-    pool = Pool(nproc)
+    pool = get_context('spawn').Pool(nproc)
     eval_results = []
     for i in range(num_classes):
         # get gt and det bboxes of this class