Tighten up mypy config (#5237)

setup.cfg

@@ -99,6 +99,10 @@ max-line-length = 120
 files = pytorch_lightning, pl_examples, benchmarks, tests
 disallow_untyped_defs = True
 ignore_missing_imports = True
+show_error_codes = True
+warn_redundant_casts = True
+warn_unused_configs = True
+warn_unused_ignores = True
 
 # todo: add proper typing to this module...
 [mypy-pytorch_lightning.callbacks.*]