Update merlin-tensorflow image to 23.08 in sagemaker example

[edknv]

This PR updates the sagemaker example to use the latest merlin-tensorflow:23.08 image. Due to changes in triton/systems,

serve file that ships with the triton container is patched and we build our container with this patched version. Specifically, lines 143-167 that were used for disallowing multiple models are removed, and also --model-control-mode=explicit is removed from the last line.

Note also that the training script train.py is updated to reflect the latest changes in the nvtabular workflow.

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[rnyak]

@jperez999 would you mind reviewing the changes? thanks!