add memory statistic (#41)

* add memory statistic

* fix import order and cuda check

mmcv/runner/hooks/logger/text.py

 import datetime
 
+import torch
+
 from .base import LoggerHook
 
 
@@ -41,5 +43,10 @@ class TextLoggerHook(LoggerHook):
             if isinstance(val, float):
                 val = '{:.4f}'.format(val)
             log_items.append('{}: {}'.format(name, val))
+        # statistic memory
+        if runner.mode == 'train' and torch.cuda.is_available():
+            mem = torch.cuda.max_memory_allocated()
+            mem_mb = int(mem / (1024 * 1024))
+            log_items.append('{}: {}'.format('memory', mem_mb))
         log_str += ', '.join(log_items)
         runner.logger.info(log_str)