[mypy] nncf/*py (#3176)

### Changes

Enable mypy check for `nncf/*py`

Author: AlexanderDokuchaev

Diff for: nncf/errors.py

@@ -136,7 +136,7 @@ class StatisticsCacheError(Exception):
     Raised when any error occurs during statistics caching.
     """
 
-    def __str__(self):
+    def __str__(self) -> str:
         return (
             "The error occurred during statistics caching:\n"
             f"{super().__str__()}\n"

Diff for: nncf/parameters.py

@@ -16,7 +16,7 @@
 
 class StrEnum(str, Enum):
     def __str__(self) -> str:
-        return self.value
+        return str(self.value)
 
 
 @api(canonical_alias="nncf.TargetDevice")

Diff for: nncf/scopes.py

@@ -141,7 +141,7 @@ def convert_ignored_scope_to_list(ignored_scope: Optional[IgnoredScope]) -> List
     :param ignored_scope: The ignored scope.
     :return: An ignored scope in the legacy format as list.
     """
-    results = []
+    results: List[str] = []
     if ignored_scope is None:
         return results
     results.extend(ignored_scope.names)
@@ -164,7 +164,7 @@ def get_matched_ignored_scope_info(
     :param nncf_graphs: Graphs.
     :returns: Matched ignored scope along with all matches.
     """
-    names, patterns, types, subgraphs_numbers = set(), set(), set(), set()
+    names, patterns, types, subgraphs_numbers = set(), set(), set(), set()  # type: ignore
     matches = {"names": names, "patterns": set(), "types": set(), "subgraphs": set()}
 
     for graph in nncf_graphs:

Diff for: pyproject.toml

@@ -105,6 +105,7 @@ files = [
     "nncf/quantization/range_estimator.py",
     "nncf/quantization/telemetry_extractors.py",
     "nncf/telemetry/",
+    "nncf/*py",
 ]
 
 [tool.ruff]