Override `callbacks.model_checkpoint.monitor` in metrics

[dxoigmn]

Metrics should automatically override callbacks.model_checkpoint.monitor. That is, there is no reason this: https://github.com/IntelLabs/MART/blob/2c62aad375e146036696e88c09f5cb3a0f7131fd/mart/configs/callbacks/model_checkpoint.yaml#L9

Shouldn't be able to be overridden when including this: https://github.com/IntelLabs/MART/blob/2c62aad375e146036696e88c09f5cb3a0f7131fd/mart/configs/experiment/COCO_TorchvisionRetinaNet.yaml#L6