Add typing to the Trainer object within LightningModules

[johnhenning]

Proposed refactor

I want to add typing for the trainer pointer within the LightningModule.

Motivation

This would improve autocompletion for IDEs.

Pitch

I would like to make the following addition:

(LightningModule)[https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/core/lightning.py#L97]

class LightningModule(...):
    ...
    def __init__(self, *args: Any, **kwargs: Any) -> None:
        ...
        self.trainer = None

changed to

class LightningModule(...):
    ...
    def __init__(self, *args: Any, **kwargs: Any) -> None:
        ...
        self.trainer: Optional[Trainer] = None

Additional context

See changes here

---

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cc @Borda

[akihironitta]

Hi @jlhbaseball15, thanks for your suggestion! We're trying to add annotations to the whole codebase in #7073. Would you be interested in submitting a PR?

[johnhenning]

Yep, I already have a branch on a fork. I'd be happy to work on more typing overall as well!