Sort imports for optional third-party libraries.

These libraries aren't always installed in the virtual environment where
isort is running. Declaring them properly avoids mixing these
third-party imports with local imports.

examples/pplm/run_pplm_discrim_train.py

@@ -26,12 +26,12 @@ import torch
 import torch.nn.functional as F
 import torch.optim as optim
 import torch.utils.data as data
-from tqdm import tqdm, trange
-
 from nltk.tokenize.treebank import TreebankWordDetokenizer
-from pplm_classification_head import ClassificationHead
 from torchtext import data as torchtext_data
 from torchtext import datasets
+from tqdm import tqdm, trange
+
+from pplm_classification_head import ClassificationHead
 from transformers import GPT2LMHeadModel, GPT2Tokenizer