Add unconditional import when type checking

[kandluis]

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.

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[ananthsub]

thanks for the fix!

[pep8speaks]

Hello @kandluis! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-03-18 16:28:08 UTC

[kandluis]

Accidentally closed. Reopening. Refactored to keep only imports in the if/else branches.

[SeanNaren]

I'll approve however not too well versed here, pinging @carmocca @kaushikb11 who may have more input.

[tchaton]

@Borda Any updates ?

[Borda]

@Borda Any updates ?

it is ready to land #6590 just need your confirmation...

[kandluis]

Thanks @Borda for merging into refactors! :)