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.

setup.cfg

@@ -3,7 +3,21 @@ ensure_newline_before_comments = True
 force_grid_wrap = 0
 include_trailing_comma = True
 known_first_party = transformers
-known_third_party = packaging
+known_third_party =
+    fairseq
+    fastprogress
+    git
+    nltk
+    packaging
+    PIL
+    psutil
+    seqeval
+    sklearn
+    tensorboardX
+    tensorflow_datasets
+    torchtext
+    torchvision
+
 line_length = 119
 lines_after_imports = 2
 multi_line_output = 3