[WIP] enabling `inference_mode` for pipelines for potentially improved perf.
Author: Narsil
Pull request: yes

# What does this PR do?

This won't work on torch==1.7.1 but does on >=1.8.1 (LTS).

Question is, should we enable this with a compatibility layer, or simply do nothing.
I think we need a bit of benchmarking to assess the value of this change first.

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Comments:
@Narsil Curious if you already observed improved performance through this mode? 

So small, not really worth it right now. (a few percent tops)

Main roadblock is that the context manager does not exist in torch 1.7 which is still supported by transformers. (So enabling it would mean adding more logic in transformers to basically use inference_mode when available else, no_grad).

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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