Remove unused line in the tutorial (#499)

I guess, the line `x_train, x_train.shape, y_train.min(), y_train.max()` was useful in notebook environment where its presence caused jupyter to display these variables.

But there are `print` statements below this line printing the same variables. So I think it's better to omit this line as it does nothing and looks confusing.

Co-authored-by: Seth Weidman <seth@sethweidman.com>
Co-authored-by: holly1238 <77758406+holly1238@users.noreply.github.com>

Author: 3 people

beginner_source/nn_tutorial.py

@@ -85,7 +85,6 @@
     torch.tensor, (x_train, y_train, x_valid, y_valid)
 )
 n, c = x_train.shape
-x_train, x_train.shape, y_train.min(), y_train.max()
 print(x_train, y_train)
 print(x_train.shape)
 print(y_train.min(), y_train.max())