Fixes bug that appears when using QA bert and distilation.
This is a fix for
https://github.com/huggingface/transformers/issues/11626
and this is a somewhat related to:
https://github.com/huggingface/transformers/issues/11941


During backward pass Pytorch complains with:
RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation
This happens because the QA model code modifies the start_positions and end_positions input tensors, using clamp_ function: as a consequence the teacher and the student both modifies the inputs, and backward pass fails.

From a quick check it looks like this is used in at least all QA code, like in:
```
cd transformers/src/transformers/models
grep -nr '[a-z]_(' . | grep clamp 

...
./xlnet/modeling_xlnet.py:1877:            start_positions.clamp_(0, ignored_index)
...
```

(and maybe for other models use.)
This may intended, but it's quite hard for the end user to track down these bugs as the issues reveal.
(And maybe PyTorch changed something and made this apparent)

It looks like I should also modify a bunch of other models (utils/check_copies.py fails) to match the same change I did in BERT, I will wait to hear from you what is the process to do so.
<|||||>It should be ok now, only non related tests are failing in run_tests_torch.
