More EpochMetric's compute_fn output types

[vfdev-5]

🚀 Feature

Following #1700 (comment) we would like to improve EpochMetric by supporting more output types returned by compute_fn:

• EpochMetric should work with compute_fn outputs: scalars, tensors, tuple/list/mapping of tensors vs currently only scalars

A code snippet on possible implementation: #1700 (comment) (It is not tested and maybe wrong, just an idea of how to do it).

It would be also helpful to raise a more user friendly error message if output type is not supported.

We need to update the docs as well.

If interested in fixing this issue, please leave a comment to assign the issue to you.

[sdesrozis]

@vfdev-5 I can help on this if needed.

[vfdev-5]

Thanks @sdesrozis , I mentioned this issue in #1729 and asked @Moh-Yakoub if he'd like to give a try...

[vfdev-5]

Detectron2 code to inspire from: https://github.com/facebookresearch/detectron2/blob/7f8f29deae278b75625872c8a0b00b74129446ac/detectron2/utils/comm.py#L148