Drop unused imports from scripts (pytorch#49956)

Summary:
Pull Request resolved: pytorch#49956

From
```
./python/libcst/libcst codemod remove_unused_imports.RemoveUnusedImportsWithGlean --no-format caffe2/
```

Test Plan: Standard sandcastle tests

Reviewed By: xush6528

Differential Revision: D25727347

fbshipit-source-id: 74d0a08aa0cfd0f492688a2b8278a0c65fd1deba

scripts/model_zoo/update-models-from-caffe2.py

@@ -6,15 +6,12 @@
 import caffe2.python.workspace as c2_workspace
 import glob
 import json
-import math
 import numpy as np
 import onnx
 import caffe2.python.onnx.frontend
 import caffe2.python.onnx.backend
 import os
 import shutil
-import subprocess
-import sys
 import tarfile
 import tempfile
 
@@ -25,7 +22,6 @@
 from caffe2.python.models.download import downloadFromURLToFile, getURLFromName, deleteDirectory
 from caffe2.proto import caffe2_pb2
 from onnx import numpy_helper
-from filechunkio import FileChunkIO
 
 
 """A script converting Caffe2 models to ONNX, and updating ONNX model zoos.

scripts/release_notes/categorize.py

@@ -1,8 +1,7 @@
-import json
 import argparse
 import os
 import textwrap
-from common import dict_to_features, categories, topics, get_features, CommitDataCache
+from common import categories, topics, CommitDataCache
 from commitlist import CommitList
 
 class Categorizer:

scripts/release_notes/commitlist.py

@@ -1,6 +1,6 @@
 import argparse
 from common import run, topics
-from collections import namedtuple, defaultdict
+from collections import defaultdict
 import os
 import csv
 import pprint