What does this PR do?

Without this change, when type checking using pyre (and possibly as well using mypy though I've not verified this) imports such as:

from pytorch_lightning import Trainer

raise errors due to the fact that they're only conditionally imported at the module level.

A workaround is to change imports to the following:

from pytorch_lightning.trainer import Trainer

However, large chunks of our documentation do not reflect this pattern. As such, this small update to the logic in the __init__.py function makes it so that when type checking (typing.TYPE_CHECKING === True) the imports are unconditional, fixing any warnings from the checkers.

This is okay since the only time the imports do not occur is when were' in the build-phase of the library.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃