Rename `checkpoint_callback` to `enable_checkpointing` in Trainer constructor

[daniellepintz]

Proposed refactoring or deprecation

Rename checkpoint_callback to enable_checkpointing in Trainer constructor

Motivation

checkpoint_callback in the Trainer constructor serves as a flag to enable/disable checkpointing. This can be confusing to users, especially since the Trainer has a checkpoint_callback property which is what is sounds like, an instance of the callback ModelCheckpoint. A more appropriate arg name would be enable_checkpointing

[carmocca]

We agree with the rename, however, there's one more question

Should we consider having disable_checkpointing instead of enable_checkpointing?

The default behaviour is to checkpoint, so any user exploring the Trainer arguments is looking to opt-out which would be more natural to do with disable.

We don't have a strong opinion on this but whatever we choose should be consistent across all true-by-default arguments.

[daniellepintz]

Great point, honestly I am a bit torn and I see both sides. Summarizing my thoughts here:

Pros for enable (heavily inspired by @rohitgr7's summary in #9754 (comment))

• enable_checkpointing requires less mental hops for the user than disable_checkpointing. e.g. with disable_checkpointing = False the user has to do two mental hops since it is a double negative (yes, that would be the default behavior so user wouldn't have to set it, but this is just to illustrate that it would add cognitive load). enable_checkpointing on the other hand is more straightforward.
• what about arguments like replace_sampler_ddp? This arg defaults to True, but I don't think we should rename it to dont_replace_sampler_ddp
• less code changes for the user (update to fewer new arg names and doesn't have to flip boolean)
• less code changes for us since enable is already the standard in other args

Pros for disable

• All boolean arguments would default to False, which makes for a simpler user experience. the user learns that any customization is always done by setting the arg to True

Stats for trainer args (including #9699):
default to True: 5
default to False: 11

I think I lean towards enable, but want to think more about it, and am interested to hear others' opinions as well.

[carmocca]

Let's go with enable