Add demo for training bert teacher on upstream task.

demo/bert/train_teacher.py

+import paddle.fluid as fluid
+from paddleslim.teachers.bert import BERTClassifier
+
+place = fluid.CUDAPlace(fluid.dygraph.parallel.Env().dev_id)
+
+with fluid.dygraph.guard(place):
+
+    bert = BERTClassifier(3)
+    bert.fit("./data/glue_data/MNLI/", 1, batch_size=32)