[FEA] Support feeding variable length multi-hot continuous columns to Candidate Retrieval models

[rnyak]

🚀 Feature request

Currently, we don’t support variable-length continuous columns out of the box, therefore, when we train Two-Tower model with variable length multi-hot continuous columns we get an error. See this GH ticket for further information.

https://github.com/NVIDIA-Merlin/Merlin/issues/550

This is a request from a customer. This is also related to that task: https://github.com/NVIDIA-Merlin/models/issues/459