add a checked parameter type in _maybe_load_yaml()

[williamjqk]

In the examples of the official manual, there are some examples whose parameters maybe passed in as a list of dicts.
e.g. the "tasks" parameter in the following example,
In https://google.github.io/seq2seq/nmt/#decoding-with-beam-search,

python -m bin.infer \
  --tasks "
    - class: DecodeText
    - class: DumpBeams
      params:
        file: ${PRED_DIR}/beams.npz" \
  --model_dir $MODEL_DIR \
  --model_params "
    inference.beam_search.beam_width: 5" \
  --input_pipeline "
    class: ParallelTextInputPipeline
    params:
      source_files:
        - $DEV_SOURCES" \
  > ${PRED_DIR}/predictions.txt