Gradient accumulation for TFTrainer
# What does this PR do?
```TFTrainer``` does not work with ```gradient_accumulation_steps``` > 1 (I am doing with ```TFGPT2LMHeadModel```).
Similar treatment of #6479 is done for labels.

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- [ ] Did you write any new necessary tests?


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 tensorflow: @jplu 
Sorry we have to revert this PR as `labels` is not a dict but a tensor and makes fails all our examples.<|||||>Thanks a lot for having spotted this cas, a more adapted fix will be available here #9616 very sorry for the inconvenience.<|||||>Never mind. Thanks again for the fix.