fix: Support multiple instances in custom predict sample

[vinnysenthil]

Fixes b/185906700 🦕

[wangminzheng]

The Sample Request on the console is not reflecting this change.
It still uses the argument of "instance_dict":

predict_custom_trained_model_sample(
project="",
endpoint_id="",
location="",
instance_dict={ "instance_key_1": "value", ...}
)