More EpochMetric's compute_fn output types

🚀 Feature

Following https://github.com/pytorch/ignite/pull/1700#issuecomment-787151203 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: https://github.com/pytorch/ignite/pull/1700#issuecomment-790413236 (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.