Fixed GMRAE implementation + Improved for DDP

[KickItLikeShika]

Fixes #2087, #1284

• Fixed GMRAE implementation to be somehow similar to EpochMetric, all the predictions and targets gathered first then compute the GMRAE on all the gathered values to avoid the misleading results because of computing the mean over a single batch.
• Updated the integration tests.
• Made the metric compatible with DDP and added the necessary tests.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[sdesrozis]

@KickItLikeShika Thank you, it looks good to me.

So finally, \bar{A} means the mean of A in the formula.

[vfdev-5]

Thanks a lot @KickItLikeShika !