Update utils.py (#616)

* Update utils.py

Example of deterministic weight initialization

* Update utils.py

tensorpack/utils/utils.py

@@ -50,6 +50,19 @@ def fix_rng_seed(seed):
 
     Note:
         See https://github.com/ppwwyyxx/tensorpack/issues/196.
+
+    Examples:
+
+        Fix random seed in both tensorpack and tensorflow.
+
+    .. code-block:: python
+
+            import tensorpack.utils.utils as utils
+
+            seed = 42
+            utils.fix_rng_seed(seed)
+            tesnorflow.set_random_seed(seed)
+            # run trainer
     """
     global _RNG_SEED
     _RNG_SEED = int(seed)