[BugFix] Fixes #20983 - Store py_func dtypes for correct conversion

1.  Implement wrapper class `PythonFunc` to store Tout list
    for appropriate dtype conversion from tensorflow types to
    numpy types using the `dtype.as_numpy_dtype` member.
2.  Adjust BUILD file to include py_test of the script_ops_test.
3.  Add script_ops_test to illustrate broken behavior (see
    `test_convert_empty_list`). Run the specific test suite with:

      bazel test //tensorflow/python:script_ops_test

4.  `_convert` was being called from dataset_ops, update this to
    refer to the `PythonFunc` instead of the `FuncRegistry`.
5.  [Review Feedback] Move script_ops tests to py_func_test.py. Also
    removed the BUILD rules for the deleted test file.