Use --no-implicit-optional for type checking (#6413)

* Use --no-implicit-optional for type checking

This is needed for PEP 484 compliant type checking (as of 2018).
mypy will change its defaults soon.

See https://github.com/python/mypy/issues/9091



* forcing CI rerun
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

mypy.ini

@@ -4,6 +4,7 @@ files = torchvision
 show_error_codes = True
 pretty = True
 allow_redefinition = True
+no_implicit_optional = True
 warn_redundant_casts = True
 
 [mypy-torchvision.prototype.features.*]

torchvision/datasets/utils.py

@@ -458,7 +458,7 @@ T = TypeVar("T", str, bytes)
 def verify_str_arg(
     value: T,
     arg: Optional[str] = None,
-    valid_values: Iterable[T] = None,
+    valid_values: Optional[Iterable[T]] = None,
     custom_msg: Optional[str] = None,
 ) -> T:
     if not isinstance(value, torch._six.string_classes):