Fix B006 lint errors: using mutable structure in default argument. (#18178)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18178
ghimport-source-id: 667ee76b418f505fa64b863e52a603c508dcd1bf

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18184 Fix B903 lint: save memory for data classes with slots/namedtuple
* #18181 Fix B902 lint error: invalid first argument.
* **#18178 Fix B006 lint errors: using mutable structure in default argument.**
* #18177 Fix lstrip bug revealed by B005 lint

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Differential Revision: D14530874

fbshipit-source-id: 38f4456a085bfe55f2a96fff53028ebd0d621604

.flake8

@@ -6,5 +6,5 @@ max-line-length = 120
 ignore =
     E203,E305,E402,E501,E721,E741,F401,F403,F405,F821,F841,F999,W503,W504,C408,
     # ignores below are temporary, fix them and remove please!
-    B006,B007,B008,B902,B903
+    B007,B008,B902,B903
 exclude = docs/src,venv,third_party,caffe2,scripts,docs/caffe2,tools/amd_build/pyHIPIFY,torch/lib/include,torch/lib/tmp_install,build,torch/include

aten/src/ATen/code_template.py

@@ -30,7 +30,10 @@ class CodeTemplate(object):
         self.pattern = pattern
         self.filename = filename
 
-    def substitute(self, env={}, **kwargs):
+    def substitute(self, env=None, **kwargs):
+        if env is None:
+            env = {}
+
         def lookup(v):
             return kwargs[v] if v in kwargs else env[v]
 

aten/src/ATen/common_with_cwrap.py

@@ -95,7 +95,10 @@ def filter_unique_options(options, allow_kwarg, type_to_signature, remove_self):
 
 
 def enumerate_options_due_to_default(declaration,
-                                     allow_kwarg=True, type_to_signature=[], remove_self=True):
+                                     allow_kwarg=True, type_to_signature=None, remove_self=True):
+
+    if type_to_signature is None:
+        type_to_signature = []
 
     # Checks to see if an argument with a default keyword is a Tensor that
     # by default can be NULL. In this case, instead of generating another