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/distillation/distiller.py

@@ -19,6 +19,7 @@ import math
 import os
 import time
 
+import psutil
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
@@ -27,7 +28,6 @@ from torch.utils.data import BatchSampler, DataLoader, RandomSampler
 from torch.utils.data.distributed import DistributedSampler
 from tqdm import tqdm
 
-import psutil
 from grouped_batch_sampler import GroupedBatchSampler, create_lengths_groups
 from lm_seqs_dataset import LmSeqsDataset
 from transformers import get_linear_schedule_with_warmup