change: merge method inputs with class inputs

[benieric]

Issue #, if available:

Description of changes:

• Small change in behavior for managing inputs in ModelTrainer.
• merge method inputs with class inputs instead of override

Previous Behavior:

model_trainer = ModelTrainer(input_data_config=[model_input]) # user provides model channel 

model_trainer.train(input_data_config[train_input]) # User provides train input, and would override the model input when calling train

New Behavior:

model_trainer = ModelTrainer(input_data_config=[model_input]) # user provides model channel 

model_trainer.train(input_data_config[train_input]) # method and class inputs merged when calling API

Testing done:

• Unit test

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