check deleted modules in dataflow/ to fix 'git pull' users

tensorpack/dataflow/__init__.py

@@ -19,8 +19,12 @@ def _global_import(name):
 
 
 __SKIP = set(['dftools', 'dataset', 'imgaug'])
+_CURR_DIR = os.path.dirname(__file__)
 for _, module_name, __ in iter_modules(
         [os.path.dirname(__file__)]):
+    srcpath = os.path.join(_CURR_DIR, module_name + '.py')
+    if not os.path.isfile(srcpath):
+        continue
     if not module_name.startswith('_') and \
             module_name not in __SKIP:
         _global_import(module_name)