Adds a multiply_grads akin to fairseq

[muellerzr]

What does this PR do?

Helps w/ DDP wrt huggingface/transformers#34283

fairseq came up with a technique to help also with countering gradient accumulation by countering grad update by multiplying the gradients by num_gpus / num_items_in_batch. This PR adds an interface for the Trainer to hook into.

Without it:

Fixes # (issue)

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@SunMarc

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[SunMarc]

Nice ! It would be also nice to compare if it is better to update the optimizer or the loss directly in transformers.

[muellerzr]

I found in general both were needed.