Autocast wrappers with a variadic keyword parameter keep supplied values
  nested under that parameter after signature binding. Flatten the mapping
  before extracting standard autocast settings so explicit values are not
  replaced by defaults.

  Test Plan:

  spin quicklint
  python test/cpp_extensions/open_registration_extension/torch_openreg/tests/test_compile.py TestAutocast
  python test/dynamo/test_ctx_manager.py CtxManagerTests.test_privateuse1_autocast_class_match CtxManagerTests.test_autocast_cpu CtxManagerTests.test__enter__exit_autocast