[Task] Add multi-gpu data parallel example

[bschifferer]

Let's create one examples for multi-GPU data parallel training for TensorFlow:

In NVTabular, we have multiple multi-GPU examples. We move them to Merlin as legacy.

• https://github.com/NVIDIA-Merlin/NVTabular/tree/main/examples/multi-gpu-movielens

PRs:

• NVIDIA-Merlin/NVTabular#1711
• NVIDIA-Merlin/Merlin#742

After we created the example, let's remove multi-gpu-movielens in the legacy folder