https://docs.aws.amazon.com/sagemaker/latest/APIReference/API_AutoMLChannel.html

This can be done now if you specify validation channel type.

A channel is a named input source that training algorithms can consume. The validation dataset size is limited to less than 2 GB. The training dataset size must be less than 100 GB. For more information, see Channel.

ChannelType
The channel type (optional) is an enum string. The default value is training. Channels for training and validation must share the same ContentType and TargetAttributeName. For information on specifying training and validation channel types, see How to specify training and validation datasets.

Type: String

Valid Values: training | validation

Required: No