Megatron positional encoding alibi fix
# What does this PR do ?

This PR fixes a bug in ALiBi implementation.

ALiBi paper: https://arxiv.org/abs/2108.12409

Results shows that ALiBi is comparable in performance to RPE, and is faster by 7% from RPE and 8% slower than learnable abs embeddings during training. 
See plots below. ALiBi (this PR) in green, ALiBi (before fix) in pink, learned absolute embeddings in red, and RPE in cyan.


![Screen Shot 2023-01-25 at 17 32 00](https://user-images.githubusercontent.com/7453913/214607208-fcc652b2-4ae0-42b5-864c-d01eb62e7996.png)
![Screen Shot 2023-01-25 at 17 32 50](https://user-images.githubusercontent.com/7453913/214607215-c41b7bc8-33e6-4378-8356-f811f22e3c02.png)
![Screen Shot 2023-01-25 at 17 33 01](https://user-images.githubusercontent.com/7453913/214607219-6bd338e0-e088-4891-ac4e-edd1fa314d16.png)
![Screen Shot 2023-01-25 at 17 33 27](https://user-images.githubusercontent.com/7453913/214607227-1480cd98-c17e-453d-bf56-5ae6f1e3d053.png)
![Screen Shot 2023-01-25 at 17 36 03](https://user-images.githubusercontent.com/7453913/214607229-f395a390-ffe8-4fd8-b55e-090623fd0b34.png)
![Screen Shot 2023-01-25 at 17 36 17](https://user-images.githubusercontent.com/7453913/214607232-6a87f3c8-aee7-4f2a-81bc-f38167932ff5.png)



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