fix a small problem in example code (microsoft#2110)

* Update mnist.py for pytorch

it's weird putting pytorch's tmp data in a folder called /tmp/tensorflow...

examples/trials/mnist-pytorch/mnist.py

@@ -128,7 +128,7 @@ def get_params():
     # Training settings
     parser = argparse.ArgumentParser(description='PyTorch MNIST Example')
     parser.add_argument("--data_dir", type=str,
-                        default='/tmp/tensorflow/mnist/input_data', help="data directory")
+                        default='/tmp/pytorch/mnist/input_data', help="data directory")
     parser.add_argument('--batch_size', type=int, default=64, metavar='N',
                         help='input batch size for training (default: 64)')
     parser.add_argument("--hidden_size", type=int, default=512, metavar='N',