Add cpu distributed fine-tuning support for transformers Trainer API
Author: kding1
Pull request: yes

Signed-off-by: Ding, Ke <ke.ding@intel.com>

# What does this PR do?
This PR adds cpu distributed fine-tuning/training support for transformers Trainer API, and it supports both MPI and oneCCL backend for CPU platform.

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@mfuntowicz 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
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Comments:
> LGTM! Thanks @kding1.
> 
> Should we add something to the documentation to highlight Distributed CPU feature in trainer?

Yes we should. where/which section would you recommend? We can do it separately.