Move param handlers to core part

[vfdev-5]

🚀 Feature

As discussed in Project-MONAI/MONAI#1949 (comment) moving https://github.com/pytorch/ignite/blob/master/ignite/contrib/handlers/param_scheduler.py to the core part: https://github.com/pytorch/ignite/tree/master/ignite/handlers could be great addition for users, apps and toolkits which do not use contrib module (e.g. MONAI).

While moving the code, we would like to ensure BC-compatibilty for at least one future release. Once the code has been moved we can put deprecation message for classes and methods inside contrib module. contrib/handlers/param_scheduler.py will be removed in v0.6.0.

A follow-up feature to this feature request could be enlarging the scope of ParamScheduler to scheduling any parameter (e.g. hyperparameters) like it is done in MONAI: Project-MONAI/MONAI#1949

cc @danieltudosiu

[danieltudosiu]

@vfdev-5 thanks for ccing.

Just to make sure I get your right.

1. For now we will just move ParamScheduler out of contrib and add MONAI ParameterScheduler to Ignite?

2. In this issue we will not aim to combine the two into a better class because we want to maintain backwards compatibility between 0.5.1 and 0.5. Right?

Thanks!

[vfdev-5]

@danieltudosiu yes, the point (2) is correct. For this issue we want only move the code and keep backward compatibility between v0.4.4 (now) up to future release v0.6.0.

In another PR associated to #1913, we would like to extend moved to core ParamScheduler to any parameter scheduler still keeping contrib API backward compatible. Your point (1).

Once we are done with both features and we released a new version, then I think we can adapt/extend MONAI's codebase if maintainers are OK with that.

[danieltudosiu]

@vfdev-5 perfect, will aim to have it done by the end of the weekend.

[vfdev-5]

Sounds great, thanks @danieltudosiu ! Let me assign this issue to you.

[vfdev-5]

Hi @danieltudosiu how things are going with this issue ?

[danieltudosiu]

@vfdev-5 sorry I had a hectic week with family issues and PhD. This week I will jump on it.

[vfdev-5]

@fco-dv please start with this issue instead of #1913 while @danieltudosiu could directly start with #1913

[fco-dv]

Yes sure @vfdev-5