Move training_trick_connector code to trainer.py

[daniellepintz]

Proposed refactoring or deprecation

Move the code from on_trainer_init in training_trick_connector to trainer.py and remove training_trick_connector
https://github.com/PyTorchLightning/pytorch-lightning/blob/c9bc10ce8473a2249ffa4e00972c0c3c1d2641c4/pytorch_lightning/trainer/connectors/training_trick_connector.py#L24

Motivation

There is not a lot of code in training_trick_connector.py and rather than maintain a separate class to slightly extend the logic in the trainer, we can simplify things by moving the logic to trainer and removing training_trick_connector.