TrainerDataLoadingMixin.request_dataloader

[daniellepintz]

1. Can we mark TrainerDataLoadingMixin.request_dataloader as protected?
   https://github.com/PyTorchLightning/pytorch-lightning/blob/dcafc95f2b0fd3f176d425139ca99676ce943a12/pytorch_lightning/trainer/data_loading.py#L566-L582

2. Can we remove the model arg from TrainerDataLoadingMixin.request_dataloader? Can we assume that whenever we call request_dataloader the Trainer already has a model attached?

Motivation

We are redesigning the Trainer's call_hook method and would like to not have the LM as an argument to the method; in order to do that we need to not pass the LM here:
https://github.com/PyTorchLightning/pytorch-lightning/blob/dcafc95f2b0fd3f176d425139ca99676ce943a12/pytorch_lightning/trainer/data_loading.py#L577