Generalize internal checks for precision plugin type, training type, accelerator type

[awaelchli]

Proposed refactor

Internally, our checks against the type of Accelerator, Precision type, strategy is not robust towards custom instances passed in by the user.

Motivation

Internally, some operations in the optimization, logging, etc. need a different code path depending on 1) Accelerator type (cpu, gpu) or 2) Precision type (apex, native) or 3) strategy type (ddp, ddp-spawn, ...). Currently we have this pattern:

if trainer._device_type == DeviceType.CPU:
    # do something only for cpu


if trainer._amp_backend == AMPType.Apex:
    # do something differently for apex

Pitch

Change these to

if isinstance(trainer.accelerator, CPUAccelerator):
    # do something only for cpu


if isinstance(trainer.precision_plugin, ApexPrecisionPlugin):
    # do something differently for apex

This has the benefits:

• User passes in custom plugins (subclasses of our plugins)
• Encapsulation: Protected members _device_type, _strategy_type, won't be abusively accessed publicly anymore. They remain an implementation detail of AcceleratorConnector
• Minimally simplifies AcceleratorConnector logic

Additional context

Discusson started in #10596

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cc @Borda @justusschock @awaelchli @rohitgr7 @kaushikb11 @akihironitta @ananthsub

[stale]

This issue has been automatically marked as stale because it hasn't had any recent activity. This issue will be closed in 7 days if no further activity occurs. Thank you for your contributions, Pytorch Lightning Team!

[tchaton]

This seems cleaner and easier to debug. Let's do it.