[WIP] LAMB optimizer

[francoishernandez]

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[DO NOT MERGE]

This is a WIP on implementing LAMB optimizer from BERT. It apparently allows to scale training on huge batches. There are some ambiguities : different algorithms between v1 and v2/v3 of the paper, some blurry definitions and no official implementation yet (a few ones are out there but differ on a few points), no clear learning_rate schedule in the paper despite detailed experiments, etc. Also, there might be some significant tuning to do in order to find appropriate values for our tasks. I open this PR for future work, when we'll have more elements.

The current version here is based on https://github.com/cybertronai/pytorch-lamb, which itself is based on torch.optimizers.Adam.

[alphadl]

LGTM