remove unnecessary imports in pytorch_tutorial.py (#1456)

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

Author: nathanbegbie

beginner_source/nlp/pytorch_tutorial.py

@@ -14,10 +14,6 @@
 # Author: Robert Guthrie
 
 import torch
-import torch.autograd as autograd
-import torch.nn as nn
-import torch.nn.functional as F
-import torch.optim as optim
 
 torch.manual_seed(1)