recommend jemalloc for gpt-neox-20b 8x

[hsubramony]

modifications to gpt-neox-20b documentation recommendation to use jemalloc instead of temalloc for this model execution

What does this PR do?

Fixes # (issue)

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• 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?

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