Support both Namespace and dict for hyperparameter saving/loading.

[awaelchli]

🚀 Feature

Let the user pass a dict to LightningModule so that after model saving it can be restored using load_from_checkpoint or load_from_metrics.

Motivation

Currently, there is nothing that prevents the user from passing in hyperparameters to LightningModule via dictionary (or even somthing else). However, the model loading/saving assumes it is always a argparse.Namespace. This could potentially be an issue when load_from_checkpoint restores the module with a Namespace passed in instead of dict.

Pitch

The model saving currently converts Namespace to dict and the model loading converts it back to Namespace.

Pitch 1: Also save the type inside the checkpoint, e.g.,

checkpoint['hparams'] = dict(hparams)
checkpoint['hparams_type'] = type(hparams)

and when restoring we instantiate with the appropriate type.

Pitch 2: Dump the whole hparams object (Namespace, dict, ...) into checkpoint without converting it to dict first and let pickle take care of the rest. Most flexible option but could give problems when loading from checkpoint.

Alternatives

Somehow restrict the user to only use argparse.

Additional context

The idea was suggested by @williamFalcon in PR #919.

[williamFalcon]

support both argparse and dict...

[awaelchli]

partially related to #525

Also, there was an attempt to pickle the hparams directly but it never got merged: #615
@neggert do you have any thoughts about this? would pickling the hparams object directly work?

[awaelchli]

implemented in #1029