Orca pytorch estimator wrapper (intel-analytics#2624)

* add estimator wrapper

* fix testcase

* fix style

* delete testcase & modify example

python/orca/example/learn/horovod/pytorch_estimator.py

@@ -22,7 +22,7 @@
 
 from zoo import init_spark_on_yarn, init_spark_on_local
 from zoo.ray import RayContext
-from zoo.orca.learn.pytorch.pytorch_horovod_estimator import PyTorchHorovodEstimator
+from zoo.orca.learn.pytorch.estimator import Estimator
 
 
 class LinearDataset(torch.utils.data.Dataset):
@@ -78,7 +78,7 @@ def validation_data_creator(config):
 
 
 def train_example():
-    trainer1 = PyTorchHorovodEstimator(
+    estimator = Estimator.from_model_creator(
         model_creator=model_creator,
         optimizer_creator=optimizer_creator,
         loss_creator=nn.MSELoss,
@@ -90,11 +90,10 @@ def train_example():
         })
 
     # train 5 epochs
-    for i in range(5):
-        stats = trainer1.train(train_data_creator)
-        print("train stats: {}".format(stats))
-        val_stats = trainer1.validate(validation_data_creator)
-        print("validation stats: {}".format(val_stats))
+    stats = estimator.fit(train_data_creator, epochs=5)
+    print("train stats: {}".format(stats))
+    val_stats = estimator.evaluate(validation_data_creator)
+    print("validation stats: {}".format(val_stats))
 
 
 parser = argparse.ArgumentParser()