Set no_merge_train_val in *_tune.yml

[Eleven1Liu]

What does this PR do?

• Allow setting no_merge_train_val in the configuration file.
• Issue: search_params.py only takes the value from arg parse. We cannot disable retrain from *_tune.yml.

# hyperparamter search
search_alg: basic_variant
embed_cache_dir: null
num_samples: 1
scheduler: null
no_merge_train_val: true # not work

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