Replace inefficient torch.sqrt taking scalar input with numpy.sqrt
# What does this PR do?

This PR fixes a performance issue for the model reformer. In model reformer running on GPUs, there are two calls for `torch.sqrt` and `torch.rsqrt` of a scalar `self.attention_head_size`. From the profiling results, we found that it calls `aten::to`, `aten::copy`, and `cudaStreamSynchronize`  etc. It is because `torch.sqrt` and `torch.rsqrt` only accept tensors as arguments, so it has to change the scalar to a tensor with only one element. By replacing them with `numpy.sqrt`, those extra operations are removed. Here is an [example script](https://gist.github.com/FindHao/abbaa7a8a0b74173d9eb2233684bb6a9) to test the impacted function. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ArthurZucker and @younesbelkada

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_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for working on this. Can you add the improvements that you have in terms of performances? (this will be useful if we look back at this code to know the motivation).<|||||>> Thanks for working on this. Can you add the improvements that you have in terms of performances? (this will be useful if we look back at this code to know the motivation).

@ArthurZucker @sgugger Thanks for your approval! Since this function call doesn't take a big portion of the whole model, the speedup at the model level is trivial. But at the function level, function `_len_and_dim_norm` obtains 27X speedup and removes CPU-GPU synchronizations.<|||||>Thanks for the number!