Minor refactors of the codebase

[akihironitta]

🚀 Refactoring

This is a tracking issue for minor refactors (which makes sure not to change existing APIs):

• pragma (see also: [coverage] Use no cover instead of no-cover #476)
  • Use # pragma: no cover instead of # pragma: no-cover.
  • Place # pragma: no cover at the beginning of clauses.
• imports
  • Add missing __all__ to __init__.py files.
  • Use if/else instead of try/except for optional import statements
  • Raise ModuleNotFoundError with helpful messages when instantiating classes or calling functions in order not to fail to import classes/functions.

Progress

• pl_bolts/callbacks/ Refactor pl_bolts.callbacks #477
• pl_bolts/datamodules/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523
• pl_bolts/datasets/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523
• pl_bolts/metrics/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523
• pl_bolts/models/ Minor refactors - cleaning models #524
• pl_bolts/models/self_supervised/ Refactor pl_bolts.models.self_supervised.amdim #479 ci: Introduce code formatter #482
• pl_bolts/optimizers/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523
• pl_bolts/transforms/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523
• pl_bolts/utils/ Minor refactors - datasets/datamodules/optimizers/transforms/metrics/utils #523

Additional context

After addressing this issue, #427 will follow up for major refactoring in models, and I'll add the importing tests in #421.