Make HF implementation match original OLMo 2 models for lower precisions
Author: 2015aroras
Pull request: yes

# What does this PR do?

Context: We found that the vLLM implementation of OLMo 2 models sometimes NaNs (https://github.com/allenai/olmo-cookbook/issues/60). Investigation revealed that this was caused by vLLM converting float32 models to float16 by default. Follow-up investigation revealed that OLMo 2 models in the original codebase do not match transformers and vLLM models in lower precisions like float16 and bfloat16.

Fix: transformers and original OLMo 2 differ in 2 ways:
1. In RMS norm, weight and hidden states are multiplied before converting back to the input dtype in the original but after in transformers.
2. Rope is applied in full precision in the original OLMo 2 rather than at the default precision. This is also true for OLMo 1.

This PR makes transformers match OLMo 2 in these manners (and OLMo 1 for point 2). This is enough to make the transformers implementation logits match the original OLMo 2's in lower precisions on CPU. This is NOT sufficient for making the transformers implementation logits match the original OLMo 1's logits in lower precisions (reason unknown), but logits still match in float32.

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Fixes #38117 


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@ArthurZucker

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Comments:
Yes, this makes sense to me but cc @arthurzucker for final approval for text models / attention!

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