[Gradient checkpoining] Correct disabling `find_unused_parameters` in Trainer when gradient checkpointing is enabled
Author: patrickvonplaten
Pull request: yes

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Following https://github.com/huggingface/transformers/pull/13657 this PR makes sure that the Trainer uses the new gradient_checkpointing logic to disable `find_unused_parameters` argument in DDP. 

@sgugger - don't think many people have switched from `self.config._gradient_checkpointing` to the new API yet, but for those that have previously `find_unused_parameters` would not have been set to `False` which then leads to some hard to debug problems like: https://discuss.pytorch.org/t/finding-the-cause-of-runtimeerror-expected-to-mark-a-variable-ready-only-once/124428/5

Not sure if this worth a patch or not


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