[fix] print self in warning. (apache#15614)

* use format

* make pylint happy

* Update block.py

python/mxnet/gluon/block.py

@@ -842,8 +842,9 @@ def hybridize(self, active=True, **kwargs):
         self._flags = list(kwargs.items())
         self._clear_cached_op()
         if active and self._forward_hooks or self._forward_pre_hooks:
-            warnings.warn('"{}" is being hybridized while still having forward hook/pre-hook. '
-                          'If "{}" is a child of HybridBlock, the hooks will not take effect.')
+            warnings.warn('"{block}" is being hybridized while still having forward hook/pre-hook. '
+                          'If "{block}" is a child of HybridBlock, the hooks will not take effect.'
+                          .format(block=self))
         super(HybridBlock, self).hybridize(active, **kwargs)
 
     def cast(self, dtype):